fix changed tags not saved in db

[phweyland]

table used_tags was not used anymore but at start time dt was still synchronizing data.tags with used_tags, removing some changes made on tags.

table used_tags is now removed.

fix issue #4034

[phweyland]

If that helps to reproduce the issue: it was visible when one changes the name / path of an existing tag.
After restarting dt the old tag is recreated.
The other changes (new tag, tag property) were kept.

[TurboGit]

Works for me!