Builtins::IsOffHeapSafe is unused now that embedded builtins are
shipped in the snapshot.

A longer explanation: there used to be a difference between
IsOffHeapSafe and IsIsolateIndependent back at an earlier stage when
we still copied off-heap-safe builtins off the heap on isolate startup.
At that point, off-heap builtins didn't need to be isolate-independent,
e.g. they could still contain embedded external references.
Now that embedded builtins are shipped in the snapshot, this
distinction is no longer needed.

Bug: v8:6666
Change-Id: I0be6120b94b01480262f472f0904ea9037941e2a
Reviewed-on: https://chromium-review.googlesource.com/1002652Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52473}