st: Clean up scroll event code

st: Clean up scroll event code

Currently the scroll event code only handles scroll events if the
adjustment's value is within the "lower" and "upper" limits. The
likely intent was to pass events to a parent scroll view when
reaching the bounds (uh, nested scroll views!), but apparently
we never made use of this, as the upper bound is actually wrong
(an adjustment's maximum value is upper - page_size, not upper).
Just handle all scroll events unconditionally and rely on the
bound checks in StAdjustment.

https://bugzilla.gnome.org/show_bug.cgi?id=672413