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Pre-MDL versions had direct relationship between LOCK_open and
LOCK_global_system_variables, e.g.:
  intern_sys_var_ptr // locks LOCK_global_system_variable
  mysql_sys_var_char
  create_options_are_valid
  ha_innobase::create
  handler::ha_create
  ha_create_table
  rea_create_table
  mysql_create_table_no_lock // locks LOCK_open
  mysql_create_table

With MDL this relationship was removed, but mutex order was still
recorded. In fact there is indirect relationship between LOCK_open
and LOCK_global_system_variables via rwlocks in reverse order.

Removed LOCK_open and LOCK_global_system_variables order recording,
instead assert that LOCK_open is never held in intern_sys_var_ptr().

This solves only one of many problems detected with MDEV-5089.