ci: drop placeholder-job tags to allow jobs to run

That tag was supposed to allow these jobs to run faster, but these
runners are currently having disk issues, and the normal runners look
like they're plenty fast enough (at least right now since almost nobody
runs ci jobs ^^).

We might revert this later, but for now let's merge this to unblock CI.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34120>

.gitlab-ci.yml

@@ -391,8 +391,6 @@ sanity:
     when: on_failure
     reports:
       junit: check-*.xml
-  tags:
-    - placeholder-job
 
 
 mr-label-maker-test:

.gitlab-ci/build/gitlab-ci.yml

@@ -832,5 +832,3 @@ python-artifacts:
   timeout: 10m
   script:
     - .gitlab-ci/prepare-artifacts-python.sh
-  tags:
-    - placeholder-job

.gitlab-ci/test/gitlab-ci.yml

@@ -31,8 +31,6 @@
   timeout: 10m
   script:
     - git diff --color=always --exit-code  # Fails if there are diffs
-  tags:
-    - placeholder-job
 
 rustfmt:
   extends:
@@ -118,8 +116,6 @@ yaml-toml-shell-py-test:
       when: on_success
     - changes: *lint_files
       when: manual
-  tags:
-    - placeholder-job
 
 .test-gl:
   extends: