Remove backwards-compatibility for allowed_{item,list}_mimetypes.
In 0.6.x, we deprecated the allowed_{item,list}_mimetypes variables, in

support of the consolidated allowed_mimetypes list. For compatibility,

we built up the new list from the old ones, if they were set. While not

a ton of work, it was extra work we didn't need to do, and extra

storage.
Support for those old lists are now gone. Clients must use

allowed_mimetypes.
Along with this, allowed_mimetypes entries no longer need to have both

an 'item' and a 'list' entry. They default to None.

Tested against the accompanying change in Review Board (/r/4809). All unit

tests pass.