vm-included-mime-types-list seems redundant

[Uday Reddy]

The purpose of vm-included-mime-types-list is unclear. It was removed by Rob at some point, but we brought it back. But the code for its use seems broken. It needs to be looked at.

[Uday Reddy (reddyuday)]

vm-included-mime-types-list was overloaded to mean two things:

1. the MIME types to be included if basic text-based inclusion is in effect.
2. to enable MIME-based inclusion if it is nil.

New variable vm-include-text-basic added in revision 1075 now controls whether text-based inclusion or MIME-based inclusion is used. (The second purpose.)

vm-included-mime-types-list now means the same thing it always did. (The first purpose.)