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