scmutil: try-delete `.hg/store/requires` if store requirements are empty

When downgrading from a shared-safe repository to non-shared-safe repository, we
end up in a case where we had requirements stored in `.hg/store/requires` but no
longer want them there.

Let's explicitly try delete the `.hg/store/requires` file if store requirements
are empty.

Differential Revision: https://phab.mercurial-scm.org/D9357

mercurial/scmutil.py

@@ -1580,6 +1580,9 b' def writereporequirements(repo, requirem'
         writerequires(repo.vfs, wcreq)
     if storereq is not None:
         writerequires(repo.svfs, storereq)
+    elif repo.ui.configbool(b'format', b'usestore'):
+        # only remove store requires if we are using store
+        repo.svfs.tryunlink(b'requires')
 
 
 def writerequires(opener, requirements):