Remove blocked feature of extra menus for project items' parent folders

Summary:
Feature has been accidentally disabled in 2013 due to porting mistake
with inverted logic in commit 0e00aa2a85254e90ec945c8c35b6aed3389edab8:
"!parent->url().isEmpty()" became wrongly "!parent->path().isValid()"

Seems no-one missed the featzure since. Besides, the UX is questionable
custom with the separate new menus popping up instead of being in submenus.

So removing as dead code for now.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6228