Remove explicit allowance for SystemUI in lock-free MessageQueue

This has been generating rare SysUI crashes in sync barrier removal.
Out of an abundance of caution, turn this off in SysUI.
We can re-enable in the new DeliQueue implementation when that's ready.

Fixes: 438822893
Flag: EXEMPT bugfix
Change-Id: I9aae321715692bf3ed3ecc84eca3301f096959f6