revlog: remove unused `rank` variable from D12209

Differential Revision: https://phab.mercurial-scm.org/D12355

mercurial/cext/revlog.c

@@ -579,7 +579,6 b' static const char *index_node(indexObjec'
 static int index_fast_rank(indexObject *self, Py_ssize_t pos)
 {
 	Py_ssize_t length = index_length(self);
-	int rank;
 
 	if (self->format_version != format_cl2 || pos >= length) {
 		return rank_unknown;