Unmarking the gcc/gcc-TEST repository as a mirror of gcc/gcc-mirror on forge

Hellos,
Richard has noticed that pull requests made on the gcc/gcc-TEST 
repository are not getting reviewers automatically assigned based on the 
.forgejo/CODEOWNERS file.

An investigation shows that this is due to the fact that gcc/gcc-TEST is 
marked as a fork of the gcc/gcc-mirror repository.

The obvious solution is to detach the two repositories in the forge.
This will have no effect on mirroring as this is taken care of by the 
batrachomyomachia bot.

There should be no downtime because of this and I plan to enact this 
change later today after consulting with the other forge admins.

Cheers,
Claudio