[WebAssembly][SelectionDAG] Remove unused WebAssemblyDAGToDAGISel::ForCodeSize.

Summary:
This follows from the discussion at D70095.

D70095 moves hasOptSize calls into SelectionDAG::shouldOptForSize to allow
querying size optimization conditions together with profile guided size
optimization.

Since it appears that size optimizations for WebAssembly SelectionDAG haven't
been implemented yet and thus ForCodeSize is unused, and it would not make a lot
of sense to call shouldOptForSize here as the necessary profile data like
PSI/BFI aren't available at this point, it seems good and less confusing to
remove this for now and use shouldOptForSize when they are implemented in the
future.

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, aheejin, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70567