patch: remove unused fsbackend._join()

The function lost its last caller in 2a095d3442e0 (patch: replace
functions in fsbackend to use vfs, 2014-06-05) when the callers
started relying on the opener to do the join.

mercurial/patch.py

@@ -456,9 +456,6 b' class fsbackend(abstractbackend):'
         super(fsbackend, self).__init__(ui)
         self.opener = vfsmod.vfs(basedir)
 
-    def _join(self, f):
-        return os.path.join(self.opener.base, f)
-
     def getfile(self, fname):
         if self.opener.islink(fname):
             return (self.opener.readlink(fname), (True, False))