[ticket/13197] Also delete group avatar from users that have it set

PHPBB3-13197
[ticket/13197] Test that user's group avatar is deleted by group avatar delete

PHPBB3-13197
[ticket/13197] Correctly format sql query

PHPBB3-13197
[ticket/13197] Fix tabbing

PHPBB3-13197
Merge pull request #3064 from marc1706/ticket/13197

[ticket/13197] Also delete group avatar from users that have it set