[Bookmarks Runner] Avoid leaking FetchSqlite instances

Summary:
The favicon and places instances are created in the prepare() call,
leaking the instance from the last invocation. Delete the old
instance in teardown().

See also T9626.

Test Plan:
run several queries in krunner
-> instances are no longer leaked

Reviewers: Frameworks, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16410