Fixes leak of DolphinFileMetaDataWidget in ToolTipManager

Fixes leak of DolphinFileMetaDataWidget in ToolTipManager

Summary:
Fixes leak of DolphinFileMetaDataWidget in ToolTipManager. The
destructor of ToolTipManager failed to delete the m_fileMetaDataWidget
member. This is seen at shutdown but also when you close a tab that has
displayed a tooltip.

Test Plan:
Compile Dolphin with address sanitizer
Open Dolphin
Show a tooltip
Close Dolphin

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19485