fix: remove vector_math dependency

[christopherfujino]

Description

Today, package:flame and package:flame_test depend on vector_math. However, the upstream Flutter SDK already depends on this package: https://github.com/flutter/flutter/blob/master/packages/flutter/pubspec.yaml#L15. Thus pub get will resolve both dependencies to a single package (and version solving fail if they are not compatible, which is why I suspect Flame uses such a wide version constraint).

This PR simplifies things by just removing the dependency from the pubspec's altogether. I ran pub get on both repos and there was no diff to the pubspec.lock, and the example app built and ran fine.

Checklist

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relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality. (no behavior change, existing tests should verify this)
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples.

Breaking Change

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• Yes, this is a breaking change. (Indicate it in the Conventional Commit prefix with a !,
  e.g. feat!:, fix!:).
• No, this is not a breaking change.

Related Issues

[luanpotter]

LGTM