ci: Don't set GIT_SUBMODULE_STRATEGY anymore

We don't use submodules anymore, so no point configuring the submodule
strategy to use.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3353>

.gitlab-ci.yml

@@ -188,8 +188,6 @@ potfile_js_check:
 build:
   stage: build
   needs: ["check_commit_log"]
-  variables:
-    GIT_SUBMODULE_STRATEGY: normal
   before_script:
     - .gitlab-ci/checkout-mutter.sh
     - meson mutter mutter/build --prefix=/usr
@@ -209,7 +207,6 @@ test:
   stage: test
   needs: ["build"]
   variables:
-    GIT_SUBMODULE_STRATEGY: normal
     XDG_RUNTIME_DIR: "$CI_PROJECT_DIR/runtime-dir"
     NO_AT_BRIDGE: "1"
   before_script:
@@ -233,8 +230,6 @@ test-coverity:
   needs: ["build"]
   stage: analyze
   allow_failure: true
-  variables:
-    GIT_SUBMODULE_STRATEGY: normal
   before_script:
     - sudo meson install -C mutter/build
   script:
@@ -287,7 +282,6 @@ dist:
   variables:
     XDG_RUNTIME_DIR: "$CI_PROJECT_DIR/runtime-dir"
     NO_AT_BRIDGE: "1"
-    GIT_SUBMODULE_STRATEGY: normal
   stage: deploy
   needs: ["build"]
   before_script: