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/foreign_keys.sql

@@ -56,8 +56,6 @@ ALTER table gene_stable_id ADD FOREIGN KEY (gene_id) REFERENCES gene(gene_id);
 
 ALTER table go_xref ADD FOREIGN KEY (object_xref_id) REFERENCES object_xref(object_xref_id);
 
-ALTER table go_xref ADD FOREIGN KEY (source_external_db_id) REFERENCES external_db(external_db_id);
-
 ALTER table identity_xref ADD FOREIGN KEY (analysis_id) REFERENCES analysis(analysis_id);
 ALTER table identity_xref ADD FOREIGN KEY (object_xref_id) REFERENCES object_xref(object_xref_id);
 

sql/table.sql

@@ -729,9 +729,7 @@ CREATE TABLE go_xref (
   linkage_type            ENUM('IC', 'IDA', 'IEA', 'IEP', 'IGI', 'IMP', 
 		               'IPI', 'ISS', 'NAS', 'ND', 'TAS', 'NR', 'RCA')
                           NOT NULL,
-  source_external_db_id   INT(10) UNSIGNED DEFAULT NULL,
   KEY (object_xref_id),
-  KEY (source_external_db_id),
   UNIQUE (object_xref_id, linkage_type)
 
 ) COLLATE=latin1_swedish_ci TYPE=MyISAM;