Fix binary name in ssh-askpass-gnome.desktop

This discrepancy causes GNOME to prompt "Allow inhibiting shortcuts" dialog.

Regarding the removal of the GenericName key: This key means a generic name for what the binary does, so that DEs can group different implementations of the same purpose together. For example, there could be many browsers, many text editors, and many calendars. I cannot think of anything good and accurate for AskPass, but the binary name itself is unlikely a good fit, since it provides no new information at all. I’m open to suggestions.