apptools.naming.ui.api.explore will not be available in Mayavi2 Python Shell

In the Mayavi2 application, if one is running with wxPython + apptools < 5.0, there is an `explore` function bound to the Python Shell. While the function exists, it does not work on wxPython 4 (hence it does not work on Python 3) as of Pyface 7.1.0. The function is simply not available on Qt.

The function is going to be removed from apptools as there are no obvious external uses apart from the one in Mayavi. So if Mayavi wants to keep the function, the code can be resurrected in Mayavi code base. Alternatively if it is not actually used by anyone (so far it seems to be available for interactive uses only), its import and binding can be removed entirely.

xref: https://github.com/enthought/apptools/issues/146, https://github.com/enthought/apptools/pull/233