ci: drop concurrency block (Gitea 1.26 dropped runs with it set)

A push that only changed Cargo.lock failed to create any Actions run while the
concurrency group was present; removing it restores reliable push triggering.
Single-dev CI doesn't need run-cancellation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -8,11 +8,6 @@ on:
   pull_request:
   workflow_dispatch:
 
-# Cancel an in-progress run when a newer commit supersedes it on the same ref.
-concurrency:
-  group: ci-${{ github.ref }}
-  cancel-in-progress: true
-
 jobs:
   check:
     runs-on: arch