ci: remove release concurrency group from on_master.yaml

The on_master workflow dispatches manual_release_beta.yaml and waits for it
to complete, but both shared the same `release` concurrency group, causing a
deadlock: the dispatched run sat in `pending` while the parent held the slot.
Beta/stable workflows still own the `release` group, so serialization between
releases is preserved.

Author: vdusek

.github/workflows/on_master.yaml

@@ -7,10 +7,6 @@ on:
     tags-ignore:
       - "**" # Ignore all tags to avoid duplicate executions triggered by tag pushes.
 
-concurrency:
-  group: release
-  cancel-in-progress: false
-
 permissions:
   contents: read