Allow removing non-existent folders from include and exclude lists

Summary:
Currently, if you add a folder to Baloo's includeFolders or excludeFolders lists, it cannot be removed again if it has been deleted. This patch fixes that for both balooctl and also the KCM.

BUG: 375370
FIXED-IN: 5.51

Test Plan:

mkdir ~/foobar-test
balooctl config add excludeFolders ~/foobar-test
rmdir ~/foobar-test
balooctl config remove excludeFolders ~/foobar-test

Without the patch, balooctl exits with an error, reports "Path does not exist", and does not remove the entry.

With the patch, balooctl exits cleanly with no error and removes the path.

Works from the KCM, too.

Reviewers: Baloo, bruns

Reviewed By: Baloo, bruns

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D15844