Removed incomplete AttrTree __delitem__ method
Removed incomplete AttrTree __delitem__ method

vascotenner/holoviews,ioam/holoviews,vascotenner/holoviews,basnijholt/holoviews,basnijholt/holoviews,mjabri/holoviews,ioam/holoviews,mjabri/holoviews,basnijholt/holoviews,ioam/holoviews,mjabri/holoviews,vascotenner/holoviews