This PR allows for passing `if_exists` options to the `remove_index`
method so that we can ignore already removed indexes. This work follows
column `if/if_not_exists` from #38352 and `:if_not_exists` on `add_index`
from #38555.

We've found this useful at GitHub, there are migrations where we don't
want to raise if an index was already removed. This will allow us to
remove a monkey patch on `remove_index`.

I considered raising after the `index_name_for_remove` method is called
but that method will raise if the index doesn't exist before we get to
execute. I have a commit that refactors this but after much
consideration this change is cleaner and more straightforward than other
ways of implementing this.

This change also adds a little extra validation to the `add_index` test.
Fix `nodoc` on edited methods.