Change LDAP's "Clear mappings" to follow standard delete routines

[blizzz]

To avoid leaving garbage in the filesystem or database and preventing potential issues as fixed with #11383.

This should not cause difficulties, as it is adviced not to do it in the production stage. Since it is a fundamental change, this need to be communicated properly and clearly and should also be extended with a password confirmation.

cc @MorrisJobke

[szaimen]

I suppose this issue is still valid?

[blizzz]

aye