the Coq CI image now contains git

I added git to the Coq CI image, only to then notice it seems you already did that. If adding git is the only difference in the coq-git image, we can get rid of that image.

Cc @janno

.gitlab-ci.yml

-image: coq-git:8.5
+image: coq:8.5
 
 stages:
   - depsquick
@@ -86,4 +86,4 @@ full:
   - master
   artifacts:
     paths:
-    - coq/ra/build-time-full.txt
\ No newline at end of file
+    - coq/ra/build-time-full.txt