[WebAssembly] Remove unimplemented-simd target feature

Now that the WebAssembly SIMD specification is finalized and engines are
generally up-to-date, there is no need for a separate target feature for gating
SIMD instructions that engines have not implemented. With this change,
v128.const is now enabled by default with the simd128 target feature.

Differential Revision: https://reviews.llvm.org/D98457