COMPAT: replace import of remove_na

[jreback]

https://travis-ci.org/pandas-dev/pandas/jobs/253985437

xref #16935

this breaks seaborn which is apparently using this private method.

So let's

• open an issue on seaborn about this (optionally they can simply use s[s.notnull()] instead of the import), prob would appreciate a PR
• add back the import to pandas.core.series and deprecate remove_na (which just can call remove_na_arraylike

[jreback]

cc @gfyoung
cc @jbrockmendel

[jbrockmendel]

Should I do this in the old PR?

[jreback]

no, new PR for this.

[gfyoung]

Issue posted on seaborn : mwaskom/seaborn#1240