Remove support for running interactive programs

This removes `mch_call_shell` code for feeding programs interactively. The
removed code was supporting interactive programs in the old GUI, but right now
we only have a terminal UI.

The code is currently safe to remove because interactive programs will just
simply take control of the terminal in cooked mode.
Remove `:shell` and `suspend_shell`

Issue #450 contains more details
Remove `mch_call_shell` GUI printing code

This isn't needed right now because spawned shells will inherit stdout by
default