CLN: remove _set_subtyp
its not used anywhere, apparently legacy leftover
https://github.com/pandas-dev/pandas/search?q=_subtyp&unscoped_q=_subtyp

@jbrockmendel : Do you intend to remove any remaining ones after this (as a follow-up)?  Or could we tackle all of them in this PR? sure > Do you intend to remove any remaining ones after this

You mean legacy methods?  Nothing actively planned, just as I come across them.  Or are you suggesting _subtyp should be removed from _internal_names? > > Do you intend to remove any remaining ones after this
> 
> You mean legacy methods? Nothing actively planned, just as I come across them. Or are you suggesting _subtyp should be removed from _internal_names?

this actually migtht be doable