[WebAssembly] Revert r342701, "Add WebAssembly to LLVM_ALL_TARGTS."

There is a memory leak which is detected in some of the sanitizer builds.
MCSymbolWasm contains SmallVectors for holding signature information,
however MCContext doesn't run destructors for MCSymbols, so in cases
where the SmallVectors heap-allocate, the memory is leaked.

llvm-svn: 342707