[palette_generator] Remove old and unused configuration file for gradlew

[reidbaker]

Remove configuration file for gradlew that no longer exists.
Pre-work for #120119.
gradlew removed in #1338 but there are ~15 instances of gradlew in flutter/packages.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [ x I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[gspencergoog]

[stuartmorgan]

I thought this file was how we specified the gradle version to use; don't we have this in every Android example?

[reidbaker]

It does if there is a gradlew file and we call gradlew. I cant find the tooling that uses the locally defined gradle. When I run gradle tasks from packages/packages/palette_genrator/example/android I get the some warnings about migration to gradle 8 but tasks show correctly. My hope/expectation was that I would see unit tests or something fail in ci so I could fix/migrate this instead.

Let me spend some time figuring out all the ways this code could be tested and make sure it passes locally.

[stuartmorgan]

flutter build creates the gradlew file. I believe flutter build and flutter run use it, and we also use it directly in the repo tooling here when running Android native unit tests. (That's why config-only will be useful; currently we have to make sure we run the unit tests after the test that everything builds)