CI: Remove cache

Gitlab.com's third-parti storage works half the time anyway, and it
takes 8min to build 3mn to get it back..

Maybe someday..

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

.gitlab-ci.yml

@@ -9,19 +9,9 @@ variables:
 
 .stable:
   image: rust:latest
-  cache:
-    key: stable
-    paths:
-      - target/
-      - ~/.cargo
 
 .nightly:
   image: rustlang/rust:nightly
-  cache:
-    key: nightly
-    paths:
-      - target/
-      - ~/.cargo
 
 .build:
   stage: build