[WebAssembly] Remove unused methods from MCWasmObjectTargetWriter

These methods looks like they were originally came from
MCELFObjectTargetWriter but they are never called by the
WasmObjectWriter.

Remove these methods meant the declaration of WasmRelocationEntry
could also move into the cpp file.

Differential Revision: https://reviews.llvm.org/D33905

llvm-svn: 304804