Delete unused global variables from mysqld.h

Several variables declared in mysqld.h appear to be old system variables
that have been left over after deprecation. Delete them using IDE
refactoring to automatically search for other uses. Most cases had no
other uses in the code.

slave_allow_batching had a test that was effectively unused, as the
result was only
-ERROR HY000: Unknown system variable 'slave_allow_batching'
so that was deleted as well.

Build and test still works without issue as expected.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services.

sql/log_event.cc

@@ -64,7 +64,6 @@
 PSI_memory_key key_memory_log_event;
 #endif
 PSI_memory_key key_memory_Incident_log_event_message;
-PSI_memory_key key_memory_Rows_query_log_event_rows_query;
 
 /**
   BINLOG_CHECKSUM variable.