Update eukaryota.pm

Removed method 'set_source_id_to_external_name' to revert back to the default behaviour which is to report the display db name in the source field of the gene description.
Merge pull request #42 from arnaudxk/master

Removed method 'set_source_id_to_external_name' to revert back to the default behaviour which is to report the display db name in the source field of the gene description.