CI: disable cache

It's a pain to make gitlab clear it on Gluon updates, so it's more
harmful than useful. Just get rid off it and accept the increased build
times.

.gitlab-ci.yml

@@ -74,11 +74,7 @@ update:
     paths:
       - gluon/output/
     expire_in: 1 day
-  cache:
-    paths:
-      - ffs-openwrt-dl-cache
-      - staging_dir
-    key: "build-$CI_COMMIT_REF_SLUG"
+  cache: []
 
 target:ath79-generic:
   extends: .target