KVM: arm64: Drop HCR_VIRT_EXCP_MASK

This was introduced in commit 0369f6a3 ("arm64: KVM: EL2 register
definitions") and for more than 10 years nobody used. Remove it.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230724142257.1551-1-yuzenghui@huawei.com