Another casualty of edc78205d5. This
particular button was actually *relying* on receiving positional events
from its entire bounding box rather than `Content`, in order for the
button to be htitable more easily, which broke as other buttons were
fixed to behave more in line with expectations.
Upon closer inspection this is another case of a weird carried-over
construction. The button doesn't really need to inherit `OsuButton` or
do any of the arcane stuff that it was doing, so it's now a plain
`OsuClickableContainer` with less `Content` hackery.
Also removes a redundant one from the setter of `Current`. If the set to
current comes with an associated value change, the bind-unbind flow that
`BindableWithCurrent` implements will handle that change anyway.