Revert "sysupgrade: print errno on failure"

This commit was taken from one of my staging branches without my
knowledge. Testing exposed problems with strerror, and the commit
message was not updated to reflect the use of strerror either.

  Failed to exec upgraded: No error information

This reverts commit 039b88f.

Signed-off-by: Stijn Tintel <[email protected]>

sysupgrade.c

@@ -19,10 +19,8 @@
 #include "sysupgrade.h"
 
 #include <ctype.h>
-#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <string.h>
 #include <unistd.h>
 
 #include <libubox/blobmsg.h>
@@ -105,7 +103,7 @@ void sysupgrade_exec_upgraded(const char *prefix, char *path,
 	execvp(argv[0], argv);
 
 	/* Cleanup on failure */
-	fprintf(stderr, "Failed to exec upgraded: %s\n", strerror(-errno));
+	fprintf(stderr, "Failed to exec upgraded.\n");
 	unsetenv("WDTFD");
 	watchdog_set_cloexec(true);
 	ret = chroot(".");