arm64/fpsimd: Remove the prototype for sve_flush_cpu_state()

The function sve_flush_cpu_state() has been removed in commit 21cdd7fd
("KVM: arm64: Remove eager host SVE state saving").

So remove the associated prototype in asm/fpsimd.h.

Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>