[WebAssembly] Remove unneeded target operand flags

[WebAssembly] Remove unneeded target operand flags

This change is in preparation for the addition of new target
operand flags for new relocation types.  Have a symbol type as part
of the flag set makes it harder to use and AFAICT these are serving
no purpose.

Differential Revision: https://reviews.llvm.org/D60014

llvm-svn: 357548