As per the latest update to the 'reference types' wasm proposal, the
nullref type is removed. Following that, all its uses in V8 were also
removed. This CL:
- Removes now dead code referencing nullref.
- Changes names of functions/exceptions containing 'nullref' to 'null'.
- Changes nullref to the corresponding nullable type in some tests.

Bug: v8:7748
Change-Id: I5b4606671d7b24dd48a45a3341e8a1c056fcd1d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2238026
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68283}