Evaluate the removal of global_variables table when using `stats` users

[renecannao]

This is a follow up from #4507 (comment)

ProxySQL may be able to reply to SELECT @@max_allowed_packet without the need for a global_variables :
See commits 13c9fe0 and 2b7d080 .

Although, the use of global_variables allows to run more queries than a simple SELECT @@max_allowed_packet , therefore we may need to continue using this table unless we find a more complex solution.