Remove useless container widget around mFolderTreeWidget.

Summary:
Clearly this was from before the search lineedit moved into FolderTreeWidget.

Found in Gammaray.

Test Plan: KMail looks fine; didn't try re-enabling the search lineedit though (didn't find the setting)

Reviewers: mlaurent, #kde_pim

Reviewed By: mlaurent, #kde_pim

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D2129