Remove CI "update" step, disable "build-debug" on master

.gitlab-ci.yml

 image: ghcr.io/cirruslabs/flutter:latest
 
 stages:
-  - update
   - build-debug
   - build-release
   - release
   - deploy
 
-update:
-  stage: update
-  except:
-    - tags
-  script:
-    - flutter packages get
-    - flutter packages upgrade
-  interruptible: true
-
 android:build-debug:
   stage: build-debug
   except:
     - tags
+    - master
   script:
     # Flutter local configuration
     - echo flutter.sdk=$FLUTTER_PATH > android/local.properties