Replace `loadmesh` with multiple dispatch

The bad definition of `loadmesh` does not appear to be called in neither this repo nor in Decapodes. So I've gone ahead and deleted it. On merging this PR, I will add another issue about deleting the `loadmesh` function entirely, and just further overloading `EmbeddedDeltaDualComplex2D` or `EmbeddedDeltaSet2D` for the appropriate mesh keys.

_Originally posted by @lukem12345 in https://github.com/AlgebraicJulia/CombinatorialSpaces.jl/pull/73#issuecomment-1987291327_