[ci] Switch Linux platform tests to LUCI (flutter#4479)

Enables the new LUCI targets, and removes the Cirrus version.

Part of flutter#114373

Author: stuartmorgan-g

.ci.yaml

@@ -507,7 +507,6 @@ targets:
       channel: stable
 
   - name: Linux_desktop platform_tests master
-    bringup: true # New target
     recipe: packages/packages
     timeout: 30
     properties:
@@ -521,7 +520,6 @@ targets:
         ]
 
   - name: Linux_desktop platform_tests stable
-    bringup: true # New target
     recipe: packages/packages
     presubmit: false
     timeout: 30

.cirrus.yml

@@ -69,20 +69,6 @@ task:
           - else
           -   ./script/tool_runner.sh federation-safety-check
           - fi
-    ### Linux desktop tasks ###
-    - name: linux-platform_tests
-      # Don't run full platform tests on both channels in pre-submit.
-      skip: $CIRRUS_PR != '' && $CHANNEL == 'stable'
-      env:
-        matrix:
-          CHANNEL: "master"
-          CHANNEL: "stable"
-      build_script:
-        - ./script/tool_runner.sh build-examples --linux
-      native_test_script:
-        - xvfb-run ./script/tool_runner.sh native-test --linux --no-integration
-      drive_script:
-        - xvfb-run ./script/tool_runner.sh drive-examples --linux --exclude=script/configs/exclude_integration_linux.yaml
 
 # Heavy-workload Linux tasks.
 # These use machines with more CPUs and memory, so will reduce parallelization