Disable fuzzing for relaxed atomics (#8909)

#8905 made a breaking change to the binary format for relaxed atomics +
multibyte array access which hasn't been done in V8 yet.

This is mostly a revert of #8664, except that relaxed atomics is left
enabled in FUZZER_ARGS
([link](https://github.com/WebAssembly/binaryen/blob/578db5d8ada17bcb2f5ecee60f9049a2b68516fa/scripts/clusterfuzz/run.py#L83)),
since other fuzzing targets (e.g. Binaryen) still support the changed
binary representation.

Part of #8904.