Remove duplicate defines introduced in initial ZFS import (r168404)

This change reduces compiler warnings by removing duplicate defines

Line numbers are from r168404 (and r284648)
#define lbolt: lines 384 and 459 (531 and 648) (original was renamed later)
#define lbolt64: lines 385 and 460 (532 and 649) (original was renamed later)
#define gethrestime_sec: lines 390 and 465 (540 and 653)
uint64_t physmem: lines 402 and 463 (561 and 651)

Reviewed by: smh, delphij
Approved by: bapt (mentor)
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D2878