WL#9678: Remove EXPLAIN PARTITIONS and EXTENDED options

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

EXTENDED and PARTITIONS flags of EXPLAIN were deprecated in 5.7. This work is to
* Remove EXTENDED and PARTITIONS keywords from EXPLAIN

This will help clean up readability of the manual:
http://dev.mysql.com/doc/refman/8.0/en/execution-plan-information.html

Having multiple formats (traditional and json) plus multiple flags does not help
readability.


F1) Remove keywords EXTENDED and PARTITIONS in EXPLAIN.