post-merge fixes:
* remove log_slow_queries_not_using_indexes, no need to create variables
  that are deprecated since the moment of creation
* rename log_slow_query_enable->log_slow_query
  no other variable uses *_enable pattern
* MDEV-29626 Assertion `self == &Sys_slow_query_log' failed in fix_log_state
* tests

Closes #2137