What is the action hook that deletes a taxonomy term from the backend? And how to retireve the term id before deleting it?

Actually, you can use multiple actions, at least this is what at the end of the wp_delete_term function, which runs when you click Delete on a taxonomy term:

do_action( 'delete_term_taxonomy', $tt_id );
do_action( 'deleted_term_taxonomy', $tt_id );
do_action( 'delete_term', $term, $tt_id, $taxonomy, $deleted_term );
do_action( "delete_$taxonomy", $term, $tt_id, $deleted_term );

The last one my be the most useful, you can create an add action like this:

add_action( "delete_favorite" ...