Per go-gitea#14531, deleting a user account will delete the user's GPG keys
from the `gpg_key` table but not from `gpg_key_import`, which causes
an error when creating an account with the same email and attempting
to re-add the same key. This commit deletes all entries from
`gpg_key_import` that match any GPG key IDs belonging to the user.

Create a new function `listGPGKeys` and replace a previous use
of `ListGPGKeys`. Thanks to @6543 for the patch.