[wasm] Remove double check for --experimental-wasm-simd flag

This was left over from the previous CL to change S128LoadMem/S128StoreMem to
use prefixed opcodes. Decoding prefixed opcodes already checks for the
prototype flag.

BUG=V8:6020

R=bbudge@chromium.org

Review-Url: https://codereview.chromium.org/2946303002
Cr-Commit-Position: refs/heads/master@{#46108}

src/wasm/function-body-decoder.cc

@@ -1615,11 +1615,9 @@ class WasmFullDecoder : public WasmDecoder {
         break;
       }
       case kExprS128LoadMem:
-        CHECK_PROTOTYPE_OPCODE(simd);
         len = DecodePrefixedLoadMem(kWasmS128, MachineType::Simd128());
         break;
       case kExprS128StoreMem:
-        CHECK_PROTOTYPE_OPCODE(simd);
         len = DecodePrefixedStoreMem(kWasmS128, MachineType::Simd128());
         break;
       default: {