This removes the awkward secondary logic.
Note that we still ship a function called `__terlar_git_prompt`
because people who picked the prompt will still be calling it - we
don't update the prompt.
This prints a description of the "host". Currently that's
`(chroot:debianchroot) $USER@$hostname`
with the chroot part when needed.
This also switches the default and terlar prompts to use it, the other
prompts have slightly different coloring or logic here.
This doesn't work with fish_config.
For terlar and pythonista, remove unnecessary color setting.
For informative+git and pythonista, move variable setup into fish_prompt
Fixes#1141
