posix: use local reference to removedirs

We have a local reference to os.removedirs in module scope, but we still used
os.removedirs inside functions. This changes util to use the local reference,
which will pave the way for combining duplicated code in future patches.

mercurial/posix.py

@@ -545,7 +545,7 b' def unlinkpath(f, ignoremissing=False):'
             raise
     # try removing directories that might now be empty
     try:
-        os.removedirs(os.path.dirname(f))
+        removedirs(os.path.dirname(f))
     except OSError:
         pass