Add support for mysql's drop index (DROP INDEX idx_a ON table_a and ALTER TABLE table_a DROP INDEX idx_a)

[vimko]

This PR adds support for mysql's drop index:

• DROP INDEX idx_a ON table_a
• ALTER TABLE table_a DROP INDEX idx_a

Addresses #1863

[iffyio]

Thanks @vimko! fyi there's a similar PR here for DROP INDEX idx_a on table_a feature, we'll probably look to land that one and rebase this on top of it

[vimko]

@iffyio Indeed, I didn't notice that. Thank you very much for reminding me.