[WebAssembly] Revert r319592 "Add support for visibility flag"

r319488 reportedly broke one of LLVM bots (ubuntu-gcc7.1-werror), and this patch
depends on that patch.
See http://lab.llvm.org:8011/builders/ubuntu-gcc7.1-werror/builds/3369 for
details.

llvm-svn: 319603