[BE] Clean up trymerge code handling flaky failures (#110133)
This is the 2nd part of https://github.com/pytorch/pytorch/pull/110054. The flaky classification has been done on Dr.CI. There is no need to download flaky rule files and do the check anymore. Some tests are also updated with new examples because we mocked the list of flaky rules there. Similar tests have been done on Dr.CI.
* [x] https://github.com/pytorch/pytorch/pull/110054
* [x] Clean up the flaky rules logic because it has already been implemented on Dr. CI
* [ ] Clean up the broken trunk logic for the same reason
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110133
Approved by: https://github.com/clee2000
