Give up on full Solarized, revert to 256color

Marks a reasonably big shift in how my colours work.
To get Solarized colours perfectly, I had to change the whole
colourscheme of the current terminal window/tab, and that affected
everything else. irssi looked awful (and theming it to fit was one of
the straws), tmux's colours became sickly, git colours were affected
too, and frankly I don't like Solarized so much as to screw everything
else up like that.

In closing support for this I'm reducing my 'workload' on an ongoing
basis, improving multiple non-Vim applications, and sacrificing only a
little colour quality.