cleanup: Deadlock checker is no longer called from signal handler

cleanup: Deadlock checker is no longer called from signal handler

Clean up a few leftovers from when the deadlock checker was called
from signal handler. We stopped doing that in commit 6753333f55, in
year 2015.

- CheckDeadLock can return a return value directly to the caller,
  there's no need to use a global variable for that.

- Remove outdated comments that claimed that CheckDeadLock "signals
  ProcSleep".

- It should be OK to ereport() from DeadLockCheck now. I considered
  getting rid of InitDeadLockChecking() and moving the workspace
  allocations into DeadLockCheck, but it's still good to avoid doing
  the allocations while we're holding all the partition locks. So just
  update the comment to give that as the reason we do the allocations
  up front.