This PR removes an assertion at the end of unit tests that ensures all mouse pointers have been removed. Instead, the MouseTracker is directly renewed without checking so.

Moreover, this PR deleted all statements that remove mouse pointers at the of a test or schedule so. These changes are not really necessary, but are made nonetheless to verify that the change is safe.

This assertion (and subsequent pointer removal) was added in #32535 to solve an issue where mouse connection was leaked between tests. However this assertion doesn't feel needed for two reasons:

1. Reconstructing MouseTracker should be enough to reset states, which has already been done.
2. We should implicitly reset states if possible to reduce the requirement for test writers.

And it turns out that removing the code does not break any tests!

Moreover, this problem is also the cause for b/227634353: The developers have some tests for a web app that can be run in a real browser for debugging. And when they do, although these tests to not refer to mouse pointers at all, any movement of the real mouse will be captured by the tests, causing the test to throw for unremoved mouse pointers.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• 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.

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