Since the improvement of overload resolution (https://crrev.com/c/1304294),
overload resolution of generics doesn't take into account existing
specializations anymore. This means that the issue of infinite recursion
when an overload of Cast for HeapObject is missing doesn't exist anymore.
Thus we can get rid of the CastHeapObject workaround.

Bug: v8:7793
Change-Id: I8442cfb81b78aaa8234bcee673647261c25f9a63
Reviewed-on: https://chromium-review.googlesource.com/c/1448324Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59263}