a48159df9c
`FEAT_MPAMv2_VID` instructions and system registers, as introduced
in change d30f18d2c, are being removed at this time, as they've been
removed from the latest Arm ARM, which doesn't preclude them returning
in some form in future.
Other system registers introduced with `FEAT_MPAMv2` are unaffected,
and these continue to be ungated, but since `+mpamv2` gating is now
empty,
I'm removing this superfluous gating code.