[BE] Clean up trymerge code handling broken trunk failures (#111520)
This is the final part of https://github.com/pytorch/pytorch/pull/110054. The broken trunk classification has been done on Dr.CI, so we can just check for that in trymerge for consistency when ghstack is used.
* [x] https://github.com/pytorch/pytorch/pull/110054
* [x] https://github.com/pytorch/pytorch/pull/110133
* [x] This PR to clean up the broken trunk logic.
One important change is that `get_classifications` doesn't need to query the jobs from Rockset for the head and merge base SHA anymore, saving a query there. The function looks a lot simpler now.
### Testing
https://github.com/pytorch/pytorch/pull/111253 had 1 broken trunk failure as detected by Dr.CI from the base commit https://hud.pytorch.org/pytorch/pytorch/commit/3eb5cae3af1207ac58f77c5ac78669e276824cb9 (valid) while trymerge didn't detect that because ghstack base commit https://hud.pytorch.org/pytorch/pytorch/commit/be8e51717411e09d8e4343c055848d434964dfb5 didn't have the same failure (miss).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/111520
Approved by: https://github.com/clee2000
