DOC: removed vendored IPython.sphinxext - take 2
This reverts commit d6049a0efb7c04996bc374f3747bab8fe4c84bac.

xref https://github.com/pandas-dev/pandas/issues/18147, re-do  https://github.com/pandas-dev/pandas/pull/18193 (which was reverted in https://github.com/pandas-dev/pandas/pull/18320 because there was still a bug in IPython, which is now fixed in the released version: https://github.com/ipython/ipython/pull/10907)


