Remove the `wasmtime` feature flag

[koute]

Followup of #12486

We've just made the wasmtime-based executor to be compiled by default. The feature flag for it is still there, so it can still be disabled if necessary.

@bkchr suggested that we should just remove it, and I agree. I don't really see much of a reason to keep it. That said, let's let it sit for a release and see if anyone complains, and if not then I'll remove it. So if you have a good reason why the feature flag should be kept then now's the time to share it.