Remove halt/poweroff/reboot.

These commands call the reboot(2) system call. On Android rebooting is done
by asking init to reboot.

Change-Id: If859d61282a32e7aa20b58f42db9f91084235b7d
diff --git a/Android.mk b/Android.mk
index 824a3b7..f08b66a 100644
--- a/Android.mk
+++ b/Android.mk
@@ -96,7 +96,6 @@
     toys/other/readahead.c \
     toys/other/readlink.c \
     toys/other/realpath.c \
-    toys/other/reboot.c \
     toys/other/rev.c \
     toys/other/rfkill.c \
     toys/other/rmmod.c \
@@ -188,7 +187,7 @@
 #        sleep swapoff swapon sync touch umount vmstat
 # obsolete?: cpio killall5 link unlink
 # non-Linux: count freeramdisk fstype nbd-client oneit vconfig
-# useless?: chvt fsfreeze halt install makedevs mkfifo mktemp partprobe pivot_root poweroff pwdx reboot rev
+# useless?: chvt fsfreeze install makedevs mkfifo mktemp partprobe pivot_root pwdx rev
 #           rfkill switch_root tty unshare
 # prefer efs2progs instead?: blkid chattr lsattr