util: Remove undocumented magic of mrafs-style logs

util: Remove undocumented magic of mrafs-style logs

The MR-AFS-style logs would always include the thread number in
log entries with the timestamp; now that we are trying to rebrand
this feature as "timestampped logs", having this bonus feature
is unexpected.

Thread ids are still used at higher log levels, as enabled by SIGTSTP.

Change-Id: Ie8c276e47a34d729ccce685ddf27bfa9e7a8f9f1
Reviewed-on: https://gerrit.openafs.org/12136
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>