chore(legacy): remove orphaned heat Problem implementation

Delete the legacy transient heat `Problem` stack that never shipped inside the
opt-in module graph.

- Drop `src/legacy/problems_heat.jl`.

src/legacy/problems_heat.jl

@@ -1,117 +0,0 @@
-# This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/HeatTransfer.jl/blob/master/LICENSE
-#
-# Heat transfer problem types - consolidated from HeatTransfer.jl
-
-"""
-    Heat
-
-3D heat transfer analysis for JuliaFEM.
-
-# Fields used in formulation
-
-- `thermal conductivity`
-- `heat source`
-- `heat flux`
-- `external temperature`
-- `heat transfer coefficient`
-
-# References
-
-- https://en.wikipedia.org/wiki/Heat_equation
-- https://en.wikipedia.org/wiki/Heat_capacity
-- https://en.wikipedia.org/wiki/Heat_flux
-- https://en.wikipedia.org/wiki/Thermal_conduction
-- https://en.wikipedia.org/wiki/Thermal_conductivity
-- https://en.wikipedia.org/wiki/Thermal_diffusivity
-- https://en.wikipedia.org/wiki/Volumetric_heat_capacity
-"""
-
-struct PlaneHeat <: FieldProblem end
-struct Heat <: FieldProblem end
-
-get_unknown_field_name(::PlaneHeat) = "temperature"
-get_unknown_field_name(::Heat) = "temperature"
-
-function assemble_elements!(problem::Problem{P}, assembly::Assembly,
-    elements::Vector{Element{M,B}}, time::Float64) where
-{M,B,P<:Union{PlaneHeat,Heat}}
-
-    bi = BasisInfo(B)
-    ndofs = length(bi)
-    Ke = zeros(ndofs, ndofs)
-    fe = zeros(ndofs)
-
-    for element in elements
-        fill!(Ke, 0.0)
-        fill!(fe, 0.0)
-        for ip in get_integration_points(element)
-            J, detJ, N, dN = element_info!(bi, element, ip, time)
-            s = ip.weight * detJ
-            k = element("thermal conductivity", ip, time)
-            Ke += s * k * dN' * dN
-            if haskey(element, "heat source")
-                f = element("heat source", ip, time)
-                fe += s * N' * f
-            end
-        end
-        if haskey(element, "temperature")
-            T = [element("temperature", time)...]
-            fe -= Ke * T
-        end
-        gdofs = get_gdofs(problem, element)
-        add!(assembly.K, gdofs, gdofs, Ke)
-        add!(assembly.f, gdofs, fe)
-    end
-
-end
-
-function assemble_elements!(problem::Problem{PlaneHeat}, assembly::Assembly,
-    elements::Vector{Element{M,B}}, time::Float64) where
-{M,B<:Union{Seg2,Seg3}}
-    return assemble_boundary_elements!(problem, assembly, elements, time)
-end
-
-function assemble_elements!(problem::Problem{Heat}, assembly::Assembly,
-    elements::Vector{Element{M,B}}, time::Float64) where
-{M,B<:Union{Tri3,Quad4,Tri6,Quad8,Quad9}}
-    return assemble_boundary_elements!(problem, assembly, elements, time)
-end
-
-function assemble_boundary_elements!(problem::Problem, assembly::Assembly,
-    elements::Vector{Element{M,B}}, time::Float64) where {M,B}
-
-    bi = BasisInfo(B)
-    ndofs = length(bi)
-    Ke = zeros(ndofs, ndofs)
-    fe = zeros(ndofs)
-
-    for element in elements
-        fill!(fe, 0.0)
-        fill!(Ke, 0.0)
-        for ip in get_integration_points(element, 2)
-            J, detJ, N, dN = element_info!(bi, element, ip, time)
-            s = ip.weight * detJ
-            if haskey(element, "heat flux")
-                g = element("heat flux", ip, time)
-                fe += s * N' * g
-            end
-            if haskey(element, "heat transfer coefficient") && haskey(element, "external temperature")
-                h = element("heat transfer coefficient", ip, time)
-                Tu = element("external temperature", ip, time)
-                Ke += s * h * N' * N
-                fe += s * N' * h * Tu
-            end
-        end
-        if haskey(element, "temperature")
-            T = [element("temperature", time)...]
-            fe -= Ke * T
-        end
-        gdofs = get_gdofs(problem, element)
-        add!(assembly.K, gdofs, gdofs, Ke)
-        add!(assembly.f, gdofs, fe)
-    end
-
-end
-
-export Heat, PlaneHeat