Use stock GMimeSession by default

Our use of GMimeSession was unneeded boilerplate, and we weren't doing
anything with it.  This simplifies and clarifies that assumption.

If we want to do anything fancier later, the examples in the gmime
source are a reasonable source to work from in defining a new
GMimeSession derivative.

Since GMimeSession is going away in GMime 2.6, though, i don't
recommend using it.