[space-maps] remove count_possibly_greater_than_one

This is only ever called with a recursive space map wrapper,
and that causes too many false positives.  False positives trigger
a copy of a metadata page, so this is a false economy.

persistent-data/space_map.h

@@ -88,8 +88,6 @@ namespace persistent_data {
 
 		virtual maybe_block find_free(span_iterator &it) = 0;
 
-		virtual bool count_possibly_greater_than_one(block_address b) const = 0;
-
 		virtual void extend(block_address extra_blocks) = 0;
 
 		struct iterator {