arm64 gic: Drop unused GICV3_IVAR_REDIST_VADDR

Now that GICV3_IVAR_REDIST is available, GICV3_IVAR_REDIST_VADDR
is unused and can be removed. Drop the define and add a comment.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D20454