WL#8057: REMOVE INFORMATION_SCHEMA STATUS tables in MySQL 8.0

REMOVE the following features:

- table INFORMATION_SCHEMA.SESSION_STATUS
- table INFORMATION_SCHEMA.SESSION_VARIABLES
- table INFORMATION_SCHEMA.GLOBAL_STATUS
- table INFORMATION_SCHEMA.GLOBAL_VARIABLES
- system variable show_compatibility_56

Cleanup the related code.

User Documentation
==================

Most of the "documentation" for this WL is removal of documentation, but these
places mention what was done:

* http://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-1.html
* http://dev.mysql.com/doc/refman/8.0/en/mysql-nutshell.html#mysql-nutshell-removals
*
http://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_show_compatibility_56