Revert "[WebAssembly] Fixup `main` signature by default"

This reverts rL345880.  It caused some test failures on the
webassembly waterfall.  e.g. binaryen2.test_mainenv fails due
the fact that `envp` ends up being undef rather than 0.

Differential Revision: https://reviews.llvm.org/D54117

llvm-svn: 346187