collectd: drop rrdtool selects

Drop indirect rrdtool selects, they're already selected by rrdtool
itself so there's no need.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>