[WebAssembly] Do not remove existing output file.

FileOutputBuffer automatically removes an existing file, so we don't
need to do that. Actually doing that is discouraged because when the
linker fails to create an output for some reason after instantiating
FileOutputBufffer, FileOutputBuffer removes a temporary file and don't
touch an existing file. That's an desired behavior from the user's
point of view.

(Internally, FileOutputBuffer writes its contents to a temporary file
and then rename it over to an existing file on commit()).

Differential Revision: https://reviews.llvm.org/D43728

llvm-svn: 326281