build: simplify maven server-id definition in deploy-on-pr-merge

The existing expression was necessary when deploying snapshots on push to master. Now we only deploy proper (i.e. non-snapshot) releases.

.github/workflows/deploy-on-pr-merge.yaml

@@ -25,7 +25,7 @@ jobs:
           java-version: 17
           distribution: 'zulu'
           cache: 'maven'
-          server-id: ${{ github.event_name == 'push' && 'matsim-snapshots' || 'matsim-releases' }} #choose mvn repo
+          server-id: 'matsim-releases'
           server-username: MAVEN_USERNAME
           server-password: MAVEN_PASSWORD