Don't reset the readline completer after each prompt

This makes it impossible to setup a custom completer. The comment says that
"we must ensure that our completer is back in place," but I don't see why. All 
that does it make it impossible to use a different completer. One can trick 
IPython into not doing this by monkeypatching ip.has_readline to False, but 
then certain features like multiline editing are no longer enabled.

See https://github.com/davidhalter/jedi/pull/321.