Revert "ci: Split base and core valgrind on different runners"

Those job are not the ones that take most time in normal run (they
where when running with --n-runs) and that leads to using more runners
which might be a problem at this point.

This reverts commit 4ad22b05a15056b9e85468af266bacc7d3758ee9.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1212>

.gitlab-ci.yml

@@ -591,10 +591,8 @@ gstreamer-full-minimal:
 
 valgrind core:
   extends: '.valgrind fedora x86_64'
-  parallel: 2
   variables:
     TEST_SUITE: "check.gstreamer\\..*"
-    EXTRA_VALIDATE_ARGS: "--valgrind --parts=${CI_NODE_TOTAL} --part-index=${CI_NODE_INDEX}"
   rules:
     - changes:
       - "*"
@@ -605,10 +603,8 @@ valgrind core:
 
 valgrind base:
   extends: '.valgrind fedora x86_64'
-  parallel: 4
   variables:
     TEST_SUITE: "check.gst-plugins-base\\..*"
-    EXTRA_VALIDATE_ARGS: "--valgrind --parts=${CI_NODE_TOTAL} --part-index=${CI_NODE_INDEX}"
   rules:
     - changes:
       - "*"