WL#821: remove thr_upgraded_concurrent_insert_lock variable

global variable  thr_upgraded_concurrent_insert_lock is definded in
mysys/thr_lock.c and is used to set priority for normal INSERT's.
REPLACE/DELETE/TRUNCATE/LOAD/LOCK/UPDATE use priority from
global_system_variables.low_priority_updates - which is correct.

Using global variable (thr_upgraded_concurrent_insert_lock) is wrong because
it's dirty hack, incorrect use of my_getopt, and when one does SET
SQL_LOW_PRIORITY_UPDATES, INSERTs won't be affected, only
REPLACE/DELETE/TRUNCATE/LOAD/LOCK/UPDATE will behave.

To do: get rid of thr_upgraded_concurrent_insert_lock, and make INSERTs to use
global_system_variables.low_priority_updates.