Get rid of the old hover labels before creating new ones
Get rid of the old hover labels before creating new ones

This is to fix a bug that multiple hover labes get added to all the tracks everytime a new track is added

Ensembl/public-plugins,muffato/public-plugins,Ensembl/public-plugins,muffato/public-plugins,Ensembl/public-plugins,muffato/public-plugins,muffato/public-plugins