Bug#37536: Thread scheduling causes performance degradation at low thread count

Deprecated --skip-thread-priority startup option as newer versions of
the server won't change the thread priorities by default.

Giving threads different priorities might yield marginal improvements
in some platforms (where it actually works) but on the other hand it
might cause significant degradation depending on the thread count and
number of processors. Meddling with the thread priorities is a not a
safe bet as it is very dependent on the behavior of the cpu scheduler
and system where MySQL is being run.

From MySQL 6.0 and up the default behavior is that of not modifying
the threads priorities.

sql/mysqld.cc:
  Deprecate --skip-thread-priority