There's only one allocator kind left post-builtin-snapshot-removal,
so the 'Default' prefix can be removed.

Bug: v8:6666, v8:7990
Change-Id: Ib3c3eeb121792708591ca7be1e30adef77d3c111
Reviewed-on: https://chromium-review.googlesource.com/c/1309638
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57236}