Remove redundant section merge hash table field

Remove redundant section merge hash table field

sec_merge_hash.size duplicates sec_merge_hash.table.count, albeit using
bfd_size_type rather than unsigned int.  The only reason to have the
duplicate field is to catch unsigned int overflows, and that can be
done easily enough when and if required.  Overflow isn't possible at
the moment.  See the needs_resize comment.

	* merge.c (sec_merge_hash): Remove "size" field.
	(NEEDS_RESIZE): Delete macro, replacing with..
	(needs_resize): ..this inline function.
	(sec_merge_resize): Rename from sec_merge_maybe_resize,
	removing redundant check.
	(sec_merge_hash_insert, sec_merge_hash_lookup): Adjust to suit.
	(sec_merge_init, merge_strings): Likewise.