Remove use_emulator flag (#130634)

MInor cleanup to remove the use_emulator flag as adding the virtual device is enough to signal use of an emulator.

Land after https://flutter-review.googlesource.com/c/recipes/+/47341

*List which issues are fixed by this PR. You must list at least one issue.*
Fixes https://github.com/flutter/flutter/issues/130522

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

.ci.yaml

@@ -1588,7 +1588,6 @@ targets:
         [
           {"dependency": "android_virtual_device", "version": "33"}
         ]
-      use_emulator: "true"
 
   - name: Linux_android android_obfuscate_test
     recipe: devicelab/devicelab_drone