External cleanup

[juliantaylor]

remove pyparsing external which is unused
remove mglob external which is only used by quarantined code

Closes gh-1838

[takluyver]

We've previously provided pyparsing for third parties to use in their own prefilter transformers, so we shouldn't just remove it with no warning. As a minimum, we should post to the ipython-user list to ask if anyone knows of code that relies on it being there.

Apart from that, removing unused code is always good. ;-)

[juliantaylor]

I don't see why ipython needs to ship 4000 lines of unused (and probably very old) code
third parties can embed it themself if they need it

[takluyver]

I agree, but we have shipped that in releases up to now. So we need to warn anyone who might have been relying on it that they can't do that as of the next release. (And theoretically, if a lot of code relied on it, we'd deprecate it for a cycle first, but I can't imagine that will happen.)

[minrk]

Fernando pinged the list, but I would be surprised if anyone tres to make the case that this should still ship in IPython.

[tomspur]

+1, great to see external/ shrinking.

[takluyver]

OK, there were no objections on the mailing list (not that I expected any), and everyone seems happy about this, so I'm merging it now. Thanks, Julian.