Fix `merging` label removal (#100433)
During regular merge process, when `GitHubPR` object is created, it does not have `merging` label and when label is added it does not update existing `GitHubPR` object either
To fix the problem, call REST API wrapper `gh_remove_label` directly. Worst case that can happen, if label is already removed at this point, is that it will be printed to the stderr, which is not rendered on HUD anyway
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100433
Approved by: https://github.com/PaliC, https://github.com/kit1980
