PC3.MenuItem: Don't animate opacity change on hover

Building on the discussion in the combobox popup styling (!1547 (merged)), we concluded that it would be good to not animate opacity in menu popups in PC3 when they're being hovered.

The reason is that it makes an assumption about the style, but we want to remain as agnostic as possible about that.

Animating the opacity also isn't consistent with Breeze application menus.