circleci: add back 'unused' and 'staticcheck' tests (#5021)

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

.circleci/config.yml

@@ -11,14 +11,11 @@ executors:
 jobs:
   test:
     executor: golang
-    resource_class: large
 
     steps:
     - checkout
     - run: make promu
-    # 'unused' and 'staticcheck' are skipped because 'go mod tidy' gives different results for Go1.11.3 and Go 1.11.4.
-    # They should be added back once CircleCI updates to 1.11.4.
-    - run: make check_license style build check_assets
+    - run: make check_license style unused staticcheck build check_assets
     - run: git diff --exit-code
     - store_artifacts:
         path: prometheus