chore(legacy): delete unused 2D contact Problem stub

Remove the standalone `Contact` boundary-problem implementation that was not
included from `Legacy.jl`.

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

src/legacy/problems_contact.jl

@@ -1,73 +0,0 @@
-# This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
-
-"""
-
-Parameters
-----------
-distval
-    a charasteristic measure to skip element pair, 0..5 => near, 10+ => far
-    5 means that distance of slave element midpoint and point to project
-    is 5 times larger than length of element
-"""
-mutable struct Contact <: BoundaryProblem
-    dimension :: Int
-    rotate_normals :: Bool
-    finite_sliding :: Bool
-    friction :: Bool
-    dual_basis :: Bool
-    use_forwarddiff :: Bool
-    forwarddiff_assemble_in_pieces :: Bool
-    minimum_active_set_size :: Int
-    distval :: Float64
-    remove_from_set :: Bool # allow removal of non-potential contact pairs
-    allow_quads :: Bool
-    remove_nodes :: Vector{Int}
-    always_in_contact :: Bool
-    update_contact_pairing :: Bool
-    iteration :: Int
-    contact_state_in_first_iteration :: Symbol
-    alpha :: Float64
-    drop_tolerance :: Float64
-    store_fields :: Vector{AbstractString}
-end
-
-function Contact()
-    default_fields = ["element area", "contact area", "weighted gap",
-        "contact pressure", "active nodes", "inactive nodes", "stick nodes",
-        "slip nodes", "complementarity condition", "contact error"]
-    return Contact(
-        -1,      # dimension
-        false,   # rotate_normals
-        false,   # finite_sliding
-        false,   # friciton
-        true,    # dual basis
-        false,   # use forwarddiff
-        false,   # when using forwarddiff, assemble interface in pieces
-        0,       # minimum active set size
-        5.0,     # distance value for contact detection
-        false,   # allow removal of non-potential contact pairs
-        false,   # allow quadrangles in contact discretization
-        [],      # remove these nodes always from set
-        false,   # mainly for debugging, do not remove inactive nodes
-        true,    # update contact pairing on each loop
-        1,       # iteration counter
-        :AUTO,   # contact state in first iteration, AUTO, INACTIVE, ACTIVE
-        0.0,     # alpha basis transform parameter
-        1.0e-9,  # drop tolerance
-        default_fields)
-end
-
-function assemble!(problem::Problem{Contact}, time::Real)
-    if problem.properties.dimension == -1
-        problem.properties.dimension = dim = size(first(problem.elements), 1)
-    end
-    dimension = Val{problem.properties.dimension}
-    finite_sliding = Val{problem.properties.finite_sliding}
-    friction = Val{problem.properties.friction}
-    use_forwarddiff = Val{problem.properties.use_forwarddiff}
-    assemble!(problem, time, dimension, finite_sliding, friction, use_forwarddiff)
-    problem.properties.iteration += 1
-end
-
-const ContactElements2D = Union{Seg2}