Remove scrollbar related workarounds from list delegates

Summary:
Now that we no longer overlap scrollbars, we can get rid of all the workarounds
needed to make the overlapping scrollbars work. This gets rid of the workarounds
in ItemDelegate and related items.

Test Plan:
Before: Too much padding on the right side

After: Correct padding

Reviewers: Plasma, ngraham, mart

Reviewed By: Plasma, ngraham, mart

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26686