clean up sourmash_args.load_dbs_and_sigs(...) with scaled/num selectors

[ctb]

As I'm going through sourmash_args.load_dbs_and_sigs(...) for #1373 and #1374, I'm seeing a good opportunity for some refactoring and simplification around providing scaled/num selectors per #1072

A few thoughts -

• if we added scaled/num selectors we could eliminate the filter_compatible_signatures check on lists of signatures;
• I think that would also remove the check_lca_db_is_compatible and check_tree_is_compatible checks;
• this would collapse the entire function down to something ...much smaller, although we might not be able to remove the conditionals on database/siglist type unless we wanted to change the notify output to just talk about total signatures loaded.