iostat: remove legacy code

Some custom iostat-source target was mistakenly left in place when the
iostat package was migrated to gentargets in commit
ee779635, back 4 years ago. This
commit removes this unnecessary custom target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>