radselect: No longer set RadicalButton status to Related on right-click

Right-clicking on a selected radical button would set its (visual)
status to Related, and thus no longer Selected, even though the
underlying radical remained selected.  Furthermore, right-clicking on it
again would have no effect; there was no way to remove the Related
status of a single button, except by left-clicking on it to toggle its
selected status.

From the comment left in ButtonGrid::radicalClicked(), the original
author seemed to have some idea in mind, but left it unimplemented.
Right-clicking therefore does nothing, except confuse the user, so let's
just disable it.

(I'm not ripping the whole thing out just yet, in case someone else want
to do something with it.)