Improve cache-friendliness of user_group->delete().

Improve cache-friendliness of user_group->delete().

Doesn't clear all possible cached entries, but this should get the ones that matter most: lookups by id, nickname, and alias. This should ensure that if a group name gets reused as a new group or alias, it should work properly.
There are some user-visible areas that aren't clear such as the 'top groups' lists on the GroupsAction sidebar; if a deleted group appears in those lists it'll go away within an hour when the cached query expires.