[libc++] Remove allocation.bench.cpp (#120767)

That benchmark isn't really useful, since it doesn't benchmark anything
from libc++ (besides `operator new`). The implementation of the
benchmark also has serious problems like the fact that it allocates an
unknown amount of memory without deallocating it.

GitOrigin-RevId: ef42e9c59aae112031d6ccbbc560d559730507c8