freebsd: remove __FBSDID macro use

With FreeBSD's switch to git the $FreeBSD$ string is no longer expanded
and they have mostly been removed upstream.  Stop using __FBSDID and
remove the no-longer needed sys/cdefs.h includes.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #15527

lib/libshare/os/freebsd/nfs.c

@@ -26,9 +26,6 @@
  * Copyright (c) 2020, 2022 by Delphix. All rights reserved.
  */
 
-#include <sys/cdefs.h>
-__FBSDID("$FreeBSD$");
-
 #include <sys/param.h>
 #include <sys/vfs.h>
 

lib/libspl/os/freebsd/mnttab.c

@@ -29,9 +29,6 @@
  * functions.
  */
 
-#include <sys/cdefs.h>
-__FBSDID("$FreeBSD$");
-
 #include <sys/param.h>
 #include <sys/mount.h>
 #include <sys/mntent.h>

lib/libzfs/os/freebsd/libzfs_zmount.c

@@ -28,9 +28,6 @@
  * This file implements Solaris compatible zmount() function.
  */
 
-#include <sys/cdefs.h>
-__FBSDID("$FreeBSD$");
-
 #include <sys/param.h>
 #include <sys/mount.h>
 #include <sys/uio.h>