[libc++] Fix sized deallocation comments in tests (#98173)

Clang 19 turned on sized deallocation *by default*, but older versions
of Clang did support sized deallocation nonetheless. This updates a few
comments and removes UNSUPPORTED annotations that shouldn't be needed in
a test that passes -fsized-deallocation directly.

NOKEYCHECK=True
GitOrigin-RevId: ad016352ec61af1df87e5e46fcfcbd1bef9ba4dd
3 files changed