change (ci): remove CARGO_HOME caching as it's still broken (#1291)

.gitlab-ci.yml

@@ -16,8 +16,6 @@ image:                             paritytech/ci-linux:production
 variables:
   GIT_STRATEGY:                    fetch
   GIT_DEPTH:                       100
-  CARGO_HOME:                      "/ci-cache/${CI_PROJECT_NAME}/cargo/${CI_JOB_NAME}"
-  SCCACHE_DIR:                     "/ci-cache/${CI_PROJECT_NAME}/sccache"
   CI_SERVER_NAME:                  "GitLab CI"
   DOCKER_OS:                       "debian:stretch"
   ARCH:                            "x86_64"