fixed bug when core db does not have contigs
fixed bug when core db does not have contigs

Removed assumption that the 3 levels of assembly are chromosome,
supercontig and contig, as there are many other species with different
coord_system sequence level (e.g. primary assembly, scaffold).

A similar error might be raised in the future regarding supercontigs,
and since 'toplevel' and 'seqlevel' are linked in the assembly table,
that part of the code has also been removed.

Ensembl/ensembl-test,Ensembl/ensembl-test