This removes two unneeded casts and an unused parameter.
Note that according to the specification, {Allocator}s need to provide
an {allocate} method receiving the size {n}, and *optionally* they can
support {allocate} with a second pointer parameter which they can use
in an unspecified manner. Since we do not use the second argument, we
should just not provide that method.

Drive-by: Remove else-after-return.

R=mlippautz@chromium.org

Bug: v8:9396
Change-Id: I776c5ae2f6652e20c9f9bfd511c41ce6ad1c1329
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1690831
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62561}