rman: Remove an outdated comment that no longer applies

Since commit 2dd1bdf1834c in 2016 the r_start and r_end fields have been
rman_res_t, which was briefly unsigned long, but commit da1b038af9f9
changed the typedef to be uintmax_t instead. C99 is also something we
assume these days.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D30808