ci: Drop "regular" build job

We already do a meson build/install when exporting extension bundles, and
now that the syntax check is handled by the CI instead of `meson test`,
the job just duplicates work.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/116

.gitlab-ci.yml

@@ -43,13 +43,6 @@ eslint:
       - ${LINT_LOG}
     when: on_failure
 
-build-shell-extensions:
-  stage: build
-  script:
-    - meson _build .
-    - ninja -C _build test install
-  <<: *only_default
-
 build-bundles:
   stage: build
   script: