arm64: Remove cpus_have_const_cap()

There are no longer any users of cpus_have_const_cap(), and therefore it
can be removed.

Remove cpus_have_const_cap(). At the same time, remove
__cpus_have_const_cap(), as this is a trivial wrapper of
alternative_has_cap_unlikely(), which can be used directly instead.

The comment for __system_matches_cap() is updated to no longer refer to
cpus_have_const_cap(). As we have a number of ways to check the cpucaps,
the specific suggestions are removed.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>