pf: Remove unused fields from pf_krule

The u_* counters are used only to communicate with userspace, as
userspace cannot use counter_u64. As pf_krule is not passed to userspace
these fields are now obsolete.

MFC after: 2 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D27759

(cherry picked from commit c7bdafe2f1b703fdf72489019edc3d6b9e5483da)