loader: Remove unneeded includes

We've not needed to include link.h in the loader for a long time. I
shoud have removed it in f38658e140 rather than going link.h ->
sys/sys_link.h. It's purely a userland shared library thing. Remove it
now, since it's not needed. Also remove a few headers that are redundant
with stand.h in this environment.

Sponsored by:		Netflix
Reviewed by:		kevans, andrew,	jhibbits
Differential Revision:	https://reviews.freebsd.org/D49859

stand/common/load_elf.c

@@ -30,11 +30,8 @@
 #include <sys/exec.h>
 #include <sys/linker.h>
 #include <sys/module.h>
-#include <sys/stdint.h>
-#include <string.h>
 #include <machine/elf.h>
 #include <stand.h>
-#include <sys/link_elf.h>
 
 #include "bootstrap.h"
 #include "modinfo.h"

stand/common/load_elf_obj.c

@@ -30,11 +30,8 @@
 #include <sys/exec.h>
 #include <sys/linker.h>
 #include <sys/module.h>
-#include <stdint.h>
-#include <string.h>
 #include <machine/elf.h>
 #include <stand.h>
-#include <sys/link_elf.h>
 
 #include "bootstrap.h"
 #include "modinfo.h"