Revert "Check that PR base is default branch in trymerge.py"

This reverts commit dc38326.

Reverted pytorch#73715 on behalf of https://github.com/malfet

.github/scripts/trymerge.py

@@ -350,8 +350,6 @@ def merge_ghstack_into(self, repo: GitRepo) -> None:
             repo.amend_commit_message(msg)
 
     def merge_into(self, repo: GitRepo, dry_run: bool = False) -> None:
-        if self.base_ref() != self.default_branch():
-            raise RuntimeError("Not a pull request into default branch.")
         # Raises exception if matching rule is not found
         find_matching_merge_rule(self, repo)
         if repo.current_branch() != self.default_branch():