rman: Remove an outdated comment that no longer applies

Since commit 2dd1bdf183 in 2016 the r_start and r_end fields have been
rman_res_t, which was briefly unsigned long, but commit da1b038af9
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

sys/kern/subr_rman.c

@@ -79,11 +79,7 @@ __FBSDID("$FreeBSD$");
 /*
  * We use a linked list rather than a bitmap because we need to be able to
  * represent potentially huge objects (like all of a processor's physical
- * address space).  That is also why the indices are defined to have type
- * `unsigned long' -- that being the largest integral type in ISO C (1990).
- * The 1999 version of C allows `long long'; we may need to switch to that
- * at some point in the future, particularly if we want to support 36-bit
- * addresses on IA32 hardware.
+ * address space).
  */
 struct resource_i {
 	struct resource		r_r;