perf/core: Drop prototype for defunct arch_perf_get_page_size()

arch_perf_get_page_size() was removed in 5.11, so this removal of
the useless prototype is long overdue.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mark Rutland <mark.rutland@arm.com>