pktgen: Delete the original date from documentation

This has been updated quite a few times since 2004, and git can
keep track of the actual date for us.
Signed-off-by: NBen Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/pktgen.txt

@@ -3,8 +3,6 @@
                   HOWTO for the linux packet generator 
                   ------------------------------------
 
-Date: 041221
-
 Enable CONFIG_NET_PKTGEN to compile and build pktgen.o either in kernel
 or as module. Module is preferred. insmod pktgen if needed. Once running
 pktgen creates a thread on each CPU where each thread has affinity to its CPU.