Remove __builtin_wasm_rethrow builtin

Summary:
Remove `__builtin_wasm_rethrow` builtin. I thought it was required to implement
`__cxa_rethrow` function in libcxxabi, but it turned out it will be using
`__builtin_wasm_throw` instead.

Reviewers: dschuff, jgravelle-google

Reviewed By: jgravelle-google

Subscribers: jfb, sbc100, jgravelle-google

Differential Revision: https://reviews.llvm.org/D37931

llvm-svn: 313402