Revert the addition of a source_external_db_id field to go_xref table,

as per email from Glenn on the core list (won't be in release 45 as it
was not declared, but is likely to be in release 46).

sql/patch_44_45_d.sql

-# patch_44_45_d.sql
-#
-# title: go_xref_source_field
-#
-# description: 
-# Add a source_external_db_id field to go_xref table
-
-ALTER TABLE `go_xref` ADD COLUMN 
-  `source_external_db_id` int(10) unsigned default NULL;
-
-ALTER TABLE `go_xref` ADD KEY (source_external_db_id);
-
-# patch identifier
-INSERT INTO meta (meta_key, meta_value) 
-VALUES ('patch', 'patch_44_45_d.sql|go_xref_source_field');