[WebAssembly] Remove temporary workaround for function bitcasts

Summary:
EM_ASM no longer is lowered as varargs in C, so this workaround is
obsolete.

Reviewers: dschuff, sunfish

Subscribers: sbc100, aheejin, llvm-commits

Differential Revision: https://reviews.llvm.org/D50859

llvm-svn: 339925