BookmarksRunner: Remove caching of profile database path

Summary:
As requested in D28196 this patch removes the caching feature.
This caching causes issues when the user has multiple profiles, because the cached value
is not updated as long as the database exists.

Depends on D28473.

Test Plan: Should compile, no changes to lookup logic made.

Reviewers: Plasma, bruns, broulik

Reviewed By: bruns

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28619