chore(sparse): remove unused SparseVector DOK prototype

Delete the experimental dictionary-backed sparse vector helper not wired into
the package entry manifest.

- Drop `src/sparse/sparsevectordok.jl`.

src/sparse/sparsevectordok.jl

@@ -1,47 +0,0 @@
-# This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/FEMSparse.jl/blob/master/LICENSE
-
-mutable struct SparseVectorDOK{Tv,Ti<:Integer} <: AbstractSparseArray{Tv,Ti,1}
-    data::Dict{Ti,Tv}
-end
-
-function SparseVectorDOK()
-    return SparseVectorDOK(Dict{Int64,Float64}())
-end
-
-function SparseVectorDOK{Tv,Ti<:Integer}(b::SparseVector{Tv,Ti})
-    I, V = findnz(b)
-    c = SparseVectorDOK()
-    add!(c, I, V)
-    return c
-end
-
-function SparseVectorDOK{T}(b::Vector{T})
-    return SparseVectorDOK(sparsevec(b))
-end
-
-function add!{Tv,Ti<:Integer}(b::SparseVectorDOK{Tv,Ti}, i::Ti, v::Tv)
-    b.data[i] = get(b, i) + v
-    return nothing
-end
-
-function add!{Tv,Ti<:Integer}(b::SparseVectorDOK{Tv,Ti}, dofs::Vector{Ti}, data::Vector{Tv})
-    @assert length(dofs) == length(data)
-    z = Tv(0)
-    for i = 1:length(dofs)
-        @inbounds b.data[dofs[i]] = Base.get(b.data, i, z) + data[i]
-    end
-    return nothing
-end
-
-function get{Tv,Ti<:Integer}(b::SparseVectorDOK{Tv,Ti}, i::Ti)
-    return Base.get(b.data, i, Tv(0))
-end
-
-function get!{Tv,Ti<:Integer}(b::SparseVectorDOK{Tv,Ti}, dofs::Vector{Ti}, data::Vector{Tv})
-    for (i, j) in enumerate(dofs)
-        data[i] = get(b, i)
-    end
-    return nothing
-end
-