tools: Remove TODO from fiptool

It is quite unlikely that this number will ever change and, if it
does need to change, we should have a good reason to do so. It
seems that this comment is now redundant.

Change-Id: I409c764080748e338e9bc5606bbdcc475213fb6e
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

tools/fiptool/tbbr_config.h

@@ -11,7 +11,6 @@
 
 #include <uuid.h>
 
-/* TODO: Update this number as required */
 #define TOC_HEADER_SERIAL_NUMBER 0x12345678
 
 typedef struct toc_entry {