arm64: Get rid of ARM64_HAS_NO_HW_PREFETCH

arm64: Get rid of ARM64_HAS_NO_HW_PREFETCH

Back in 2016, it was argued that implementations lacking a HW
prefetcher could be helped by sprinkling a number of PRFM
instructions in strategic locations.

In 2023, the one platform that presumably needed this hack is no
longer in active use (let alone maintained), and an quick
experiment shows dropping this hack only leads to a 0.4% drop
on a full kernel compilation (tested on a MT30-GS0 48 CPU system).

Given that this is pretty much in the noise department and that
it may give odd ideas to other implementers, drop the hack for
good.

Suggested-by: Will Deacon <will@kernel.org>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20231122133754.1240687-1-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>