Remove DataFrame.delevel
Part of #6581.  Remove DataFrame.delevel which was previously deprecated in version 0.7.
@jreback 

This displayed issue wrong: http://pandas-docs.github.io/pandas-docs-travis/whatsnew.html#deprecations

any idea where the original deprecation came from?

if you can find it great, if not let's just remove the reference (to #420)

thanks
 I am going to take it out (so if you find the correct reference pls do a PR to add back)
