Old theme symlink is not removed

[kaidesu]

Describe the issue

When an active theme is deleted and a new one is set, the old public theme symlink becomes broken causing a 500 error.

To reproduce

1. Generate a theme
2. Activate it
3. Delete it from the filesystem
4. Attempt to activate another theme

Expected behavior

The old symlink to be cleared out

Screenshots

Version and browser

Version: v6.0.0 Beta 7

Bug report checklist

• I have filled out as much information as I can
• I have detailed how the issue can be reproduced
• I have searched for existing issues and to the best of my knowledge this is not a duplicate