[WebAssembly] Revert r288447.

Revert r288447 which introduced -mdirect. It turns out we don't need a
custom flag for this, as the information we need is in the target triple.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288604 91177308-0d34-0410-b5e6-96231b3b80d8