Correctly remember callback status when sleeping.

If a callback triggered right away after a sceKernelSleepThreadCB(), it
would trigger the "current callback" and remember callback status as
false, instead of true (since it was set later.)

This corrects that by ignoring it if both are set.
Remove hleAfterCheckAllCallbacks().

Not being used, doesn't make sense - rescheduling is what does that, only.
Merge pull request #6447 from unknownbrackets/callbacks-fix

Fix callback status forgotten by sceKernelSleepThreadCB() and others