[InformationPanel] Remove obsolete intermediate wrapper widget/layout

The Baloo::MetaDataWidget is now able to stretch in a meaningful way
by itself, remove the wrapper.

Summary: Depends on D20667

Reviewers: Dolphin, Baloo, Frameworks, ngraham, astippich, elvisangelaccio

Reviewed By: Dolphin, Baloo, ngraham, elvisangelaccio

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D20668