CI: Remove cache where not needed

The specialized Docker images don't profit from using the portage tree
cache. Thus, this commit removes the usage of the cache for these jobs.

Signed-off-by: Adrian 'nex' Schollmeyer <adrian.schollmeyer@fem.tu-ilmenau.de>

.gitlab-ci.yml

@@ -10,10 +10,6 @@ repoman_all:
   stage: repoman
   script:
     - repoman full -x -j4
-  cache:
-    key: fem-overlay-gentoo-stage3
-    paths:
-      - ./tmp
   needs: []
   except: [merge_requests]
 
@@ -22,10 +18,6 @@ repoman_commits:
   stage: repoman
   script:
     - ./tools/repoman_changed_files.sh
-  cache:
-    key: fem-overlay-gentoo-stage3
-    paths:
-      - ./tmp
   needs: []
   only: [merge_requests]
 
@@ -49,10 +41,6 @@ pkgcheck_all:
   stage: pkgcheck
   script:
     - pkgcheck scan --exit --checks="-ManifestCheck"
-  cache:
-    key: fem-overlay-gentoo-stage3
-    paths:
-      - ./tmp
   needs: []
   except: [merge_requests]
 
@@ -61,9 +49,5 @@ pkgcheck_commits:
   stage: pkgcheck
   script:
     - pkgcheck scan --exit --commits origin/master..HEAD
-  cache:
-    key: fem-overlay-gentoo-stage3
-    paths:
-      - ./tmp
   needs: []
   only: [merge_requests]