mkimage: Drop unused OPT_STRING constant

The actual opt string is inlined - and different. Seems this was a
left-over from older versions of 603e26f76346.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/mkimage.c b/tools/mkimage.c
index fbe883c..a4844d0 100644
--- a/tools/mkimage.c
+++ b/tools/mkimage.c
@@ -146,7 +146,6 @@
 	return 0;
 }
 
-#define OPT_STRING "a:A:b:B:c:C:d:D:e:Ef:Fk:i:K:ln:N:p:O:rR:qstT:vVx"
 static void process_args(int argc, char **argv)
 {
 	char *ptr;