arm64: entry: Remove confusing comment

The comment about SYS_MEMBARRIER_SYNC_CORE relying on ERET being
context-synchronizing is confusing and misplaced with kpti. Given that
this is already documented under Documentation/ (see arch-support.txt
for membarrier), remove the comment altogether.

Signed-off-by: Will Deacon <will.deacon@arm.com>