read.c: use string hash from hash.h for po_hash

po_hash code duplicates the str_hash code in hash.h for no good reason.

	* read.c (struct po_entry, po_entry_t): Delete.
	(hash_po_entry, eq_po_entry, po_entry_alloc, po_entry_find): Delete.
	(pop_insert): Use str_hash_insert.
	(pobegin): Use str_htab_create.
	(read_a_source_file, s_macro): Use str_hash_find.