Tidy up comments for terminal colours

Removes a bunch of commented commands for the Solarized theme in iTerm,
which I've now given up on.

Looks a lot neater, though.
Delete <leader>b to toggle background light/dark

Didn't work properly and I don't use it.
Add commented-out buffer number show in Airline

May be useful for :b <number> in the future. Writing it down here
because I've looked for it before and failed.
Delete cpoptions $

It was on trial and I decided I didn't like it.
Install EasyMotion plugin

I've been using it *occasionally*. It's still on trial. Need to go back
at some point and reread the docs to revise what it can do.