gdb: remove TYPE_FIELD macro

Replace all uses of it by type::field.

Note that since type::field returns a reference to the field, some spots
are used to assign the whole field structure.  See ctfread.c, function
attach_fields_to_type, for example.  This is the same as was happening
with the macro, so I don't think it's a problem, but if anybody sees a
really nicer way to do this, now could be a good time to implement it.

gdb/ChangeLog:

	* gdbtypes.h (TYPE_FIELD): Remove.  Replace all uses with
	type::field.

gdb/guile/scm-type.c

@@ -514,7 +514,7 @@ tyscm_field_smob_to_field (field_smob *f_smob)
   /* This should be non-NULL by construction.  */
   gdb_assert (type->fields () != NULL);
 
-  return &TYPE_FIELD (type, f_smob->field_num);
+  return &type->field (f_smob->field_num);
 }
 
 /* Type smob accessors.  */