MDEV-13375 back_log ignored

doing SYSVAR_AUTOSIZE() because of back_log > max_connections
enabled "autosized" flag, and that made IS_SYSVAR_AUTOSIZE()
true, which triggered the second SYSVAR_AUTOSIZE.

Remove back_log <= max_connections limit, back_log
doesn't *always* have to be smaller than max_connections.

mysql-test/suite/sys_vars/r/back_log_basic.result

@@ -1,21 +1,3 @@
 select @@global.back_log;
 @@global.back_log
-80
-select @@session.back_log;
-ERROR HY000: Variable 'back_log' is a GLOBAL variable
-show global variables like 'back_log';
-Variable_name	Value
-back_log	80
-show session variables like 'back_log';
-Variable_name	Value
-back_log	80
-select * from information_schema.global_variables where variable_name='back_log';
-VARIABLE_NAME	VARIABLE_VALUE
-BACK_LOG	80
-select * from information_schema.session_variables where variable_name='back_log';
-VARIABLE_NAME	VARIABLE_VALUE
-BACK_LOG	80
-set global back_log=1;
-ERROR HY000: Variable 'back_log' is a read only variable
-set session back_log=1;
-ERROR HY000: Variable 'back_log' is a read only variable
+1000

mysql-test/suite/sys_vars/t/back_log_basic.opt

@@ -0,0 +1 @@
+--back-log=1000 --max-connections=300

mysql-test/suite/sys_vars/t/back_log_basic.test

@@ -2,18 +2,3 @@
 # show the global and session values;
 #
 select @@global.back_log;
---error ER_INCORRECT_GLOBAL_LOCAL_VAR
-select @@session.back_log;
-show global variables like 'back_log';
-show session variables like 'back_log';
-select * from information_schema.global_variables where variable_name='back_log';
-select * from information_schema.session_variables where variable_name='back_log';
-
-#
-# show that it's read-only
-#
---error ER_INCORRECT_GLOBAL_LOCAL_VAR
-set global back_log=1;
---error ER_INCORRECT_GLOBAL_LOCAL_VAR
-set session back_log=1;
-

sql/mysqld.cc

@@ -9674,8 +9674,6 @@ static int get_options(int *argc_ptr, char ***argv_ptr)
 #endif
 
   /* Ensure that some variables are not set higher than needed */
-  if (back_log > max_connections)
-    SYSVAR_AUTOSIZE(back_log, max_connections);
   if (thread_cache_size > max_connections)
     SYSVAR_AUTOSIZE(thread_cache_size, max_connections);