Remove implication: girl_on_top

Currently the following sexual positions implicate girl_on_top (though there may be others):

• cowgirl_position
• reverse_cowgirl_position

The problem occurs when we start applying the tags for yaoi/newhalf/futanari images.
Additionally, I would prefer not to make up tags like "cowboy_position".

This isn't a problem for futanari, we already call them girls for the sake of the 1girl/multiple_girls tags.

But I agree with removing the implication for the other reasons, as cowgirl technically refers to the receiving partner being on top, not necessarily the woman.

(I changed the cowgirl_position and reverse_cowgirl_position wikis to reflect this).

I've removed the implications.