CI: rm unit-test-race step since its now coverd by unit-test too (#16856)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

.drone.yml

@@ -218,17 +218,6 @@ steps:
       GITHUB_READ_TOKEN:
         from_secret: github_read_token
 
-  - name: unit-test-race
-    image: golang:1.17
-    commands:
-      - make test-backend
-    environment:
-      GOPROXY: off
-      TAGS: sqlite sqlite_unlock_notify
-      RACE_ENABLED: true
-      GITHUB_READ_TOKEN:
-        from_secret: github_read_token
-
   - name: unit-test-gogit
     pull: always
     image: golang:1.17