Blender Git Loki

UI: Remove X axis panel dragging

X axis panel dragging traces back to Blender versions before 2.5,
where panels could be aligned horizontally. But for many years now
panels have been vertically aligned. Considering this, keeping the
X axis dragging around is a bit odd. It makes interaction confusing,
or at least more complicated. It also looks bad, since any part of
the panel outside the region is cropped.

Differential Revision: https://developer.blender.org/D9549