You can try any one of these:
pkg clean # cleans /var/cache/pkg/
rm -rf /var/cache/pkg/* # just remove it all
pkg update -f # forces update of repository catalog
rm /var/db/pkg/repo-*.sqlite # removes all remote repository catalogs
pkg bootstrap -f # forces reinstall of pkg
You can delete quite a lot but whatever you do, do NOT delete /var/db/pkg/local.sqlite. It contains the database with your installed packages. If you remove it the system will think you have nothing installed.