KVM: arm64: Remove ad-hoc CPTR manipulation from fpsimd_sve_sync()

KVM: arm64: Remove ad-hoc CPTR manipulation from fpsimd_sve_sync()

There's no need for fpsimd_sve_sync() to write to CPTR/CPACR. All
relevant traps are always disabled earlier within __kvm_vcpu_run(), when
__deactivate_cptr_traps() configures CPTR/CPACR.

With irrelevant details elided, the flow is:

handle___kvm_vcpu_run(...)
{
	flush_hyp_vcpu(...) {
		fpsimd_sve_flush(...);
	}

	__kvm_vcpu_run(...) {
		__activate_traps(...) {
			__activate_cptr_traps(...);
		}

		do {
			__guest_enter(...);
		} while (...);

		__deactivate_traps(....) {
			__deactivate_cptr_traps(...);
		}
	}

	sync_hyp_vcpu(...) {
		fpsimd_sve_sync(...);
	}
}

Remove the unnecessary write to CPTR/CPACR. An ISB is still necessary,
so a comment is added to describe this requirement.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250617133718.4014181-5-mark.rutland@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>