[ticket/12822] Remove "None" option from avatar select boxes

This option is just used for deleting avatars for which we already have
a checkbox.

PHPBB3-12822
[ticket/12822] Add method for deleting avatars and use confirm_box

The method will take care of the actual deletion process in the avatar manager.
Additionally, we'll be using a confirm box while deleting the avatar to prevent
users from accidentally deleting their avatars.

PHPBB3-12822
[ticket/12822] Fix test runs of avatar manager_test

PHPBB3-12822
Merge remote-tracking branch 'github-marc1706/ticket/12822' into develop-ascraeus

* github-marc1706/ticket/12822:
  [ticket/12822] Fix test runs of avatar manager_test
  [ticket/12822] Add method for deleting avatars and use confirm_box
  [ticket/12822] Remove "None" option from avatar select boxes