Revert "Partial fix of a bug concerning scrollbar"

This PR breaks resizing of panels with a scrollbar if they contain
multiple elements. The original commit also admits that the fix
was only partial, as the widget borders did not draw properly.

This reverts commit 5eb143aae2b200429c9181647be855f1f5a4b565.
Merge pull request #134 from enthought/fix/revert-scrollbar-policy-change

Revert "Partial fix of a bug concerning scrollbar"