[ci] Remove unused Chromium setup (flutter#4437)

The tasks that used the Cirrus template and shell script to install a specific version of Chromium have been migrated to LUCI, so this is cruft; I forgot to remove it when finishing that migration.

Also combines two setup steps into one template now that they are only used together.

Author: stuartmorgan-g

.cirrus.yml

@@ -6,23 +6,7 @@ env:
   CHANNEL: "master" # Default to master when not explicitly set by a task.
   PLUGIN_TOOL_COMMAND: "dart ./script/tool/bin/flutter_plugin_tools.dart"
 
-install_chrome_linux_template: &INSTALL_CHROME_LINUX
-  env:
-    CHROME_NO_SANDBOX: true
-    CHROME_DOWNLOAD_DIR: /tmp/chromium
-    CHROME_EXECUTABLE: $CHROME_DOWNLOAD_DIR/chrome-linux/chrome
-    CHROMEDRIVER_EXECUTABLE: $CHROME_DOWNLOAD_DIR/chromedriver/chromedriver
-    PATH: $PATH:$CHROME_DOWNLOAD_DIR/chrome-linux
-  install_chromium_script:
-    # Install a pinned version of Chromium and its corresponding ChromeDriver.
-    # Setting CHROME_EXECUTABLE above causes this version to be used for tests.
-    - ./script/install_chromium.sh
-
-tool_setup_template: &TOOL_SETUP_TEMPLATE
-  tool_setup_script:
-    - .ci/scripts/prepare_tool.sh
-
-flutter_upgrade_template: &FLUTTER_UPGRADE_TEMPLATE
+setup_template: &SETUP_TEMPLATE
   upgrade_flutter_script:
     # Channels that are part of our normal test matrix use a pinned,
     # auto-rolled version to prevent out-of-band CI failures due to changes in
@@ -45,13 +29,14 @@ flutter_upgrade_template: &FLUTTER_UPGRADE_TEMPLATE
     - fi
     # Run doctor to allow auditing of what version of Flutter the run is using.
     - flutter doctor -v
-  << : *TOOL_SETUP_TEMPLATE
+  tool_setup_script:
+    - .ci/scripts/prepare_tool.sh
 
 # Light-workload Linux tasks.
 # These use default machines, with fewer CPUs, to reduce pressure on the
 # concurrency limits.
 task:
-  << : *FLUTTER_UPGRADE_TEMPLATE
+  << : *SETUP_TEMPLATE
   gke_container:
     dockerfile: .ci/Dockerfile
     builder_image_name: docker-builder-linux # gce vm image
@@ -129,7 +114,7 @@ task:
 # These use machines with more CPUs and memory, so will reduce parallelization
 # for non-credit runs.
 task:
-  << : *FLUTTER_UPGRADE_TEMPLATE
+  << : *SETUP_TEMPLATE
   gke_container:
     dockerfile: .ci/Dockerfile
     builder_image_name: docker-builder-linux # gce vm image