[Unowned Data] Fix type-safety tests.

The implementation of UnownedUserDataInterface (which was provided for
convenience but only actually used in tests) did an invalid cast in its
constructor which was picked up by specific safety tests on Linux that
enabled RTTI.

Rather than attempting to fix this by inverting the CRDP pattern or
doing some clever workaround, this CL opts to simply remove the
"convenience" class until a better solution can be created.

Affected tests have been re-enabled (and in one case, removed).

Bug: 428749341
Change-Id: I1c16ea3b1c4c526576c5cc740b8789820449142e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6700627
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1481852}