Blender Git Loki

UI: Remove confusing "Unset" context menu entry

This was added in 37b82a2d260e, doesn't have much purpose and doesn't even do
anything in the vast majority of cases. It only affects custom properties. So
at the very least it shouldn't be shown for regular RNA property buttons. But
even for cases where it may do something, we couldn't find a good use-case. If
this operator has use-cases with some add-ons, the add-ons can expose it
differently, e.g. with a little 'x' icon next to the button, like it's done in
the keymap editor.

So removing the context menu entry now. Should this turn out to be a problem,
it can be brought back but much more selectively (only where it actually does
something). Or we could combine it with "Reset to Default".

Differential Revision: https://developer.blender.org/D8727

Reviewed by: Brecht Van Lommel, Hans Goudey