arm64: mte: remove an ISB on kernel exit

This ISB is unnecessary because we will soon do an ERET.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I69f1ee6bb09b1372dd744a0e01cedaf090c8d448
Link: https://lore.kernel.org/r/20201203073458.2675400-1-pcc@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>