Remove packages/platform from dependabot config (flutter#8099)

Package was removed in flutter#6898.

However the path was still being passed to dependabot, which made it mad:
```
ERROR <job_916983349> Error during file fetching; aborting: No files found in /packages/platform/example/android/app
```
https://github.com/flutter/packages/actions/runs/11845225387/job/33010210199

I don't understand why this only started failing on flutter#8044 and not when the package was removed (since it moved that path from one place to another https://github.com/flutter/packages/pull/8036/files#diff-dd4fbda47e51f1e35defb9275a9cd9c212ecde0b870cba89ddaaae65c5f3cd28L571)

.github/dependabot.yml

@@ -47,7 +47,6 @@ updates:
       - /packages/palette_generator/example/android/app
       - /packages/path_provider/path_provider/example/android/app
       - /packages/path_provider/path_provider_android/example/android/app
-      - /packages/platform/example/android/app
       - /packages/pigeon/example/app/android/app
       - /packages/pigeon/platform_tests/test_plugin/example/android/app
       - /packages/pigeon/platform_tests/alternate_language_test_plugin/example/android/app