arm: mvebu: db-88f6820: Drop obsolete binary.0 placeholder

It has been superseded in kwbimage.cfg in favor of an SPL in commit
9e30b31d20f0b793465d07f056b3d9885f578c0d (arm: mvebu: db-88f6820: Add
SPL support with DDR init code). Found via code review.

Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Roese <sr@denx.de>

board/Marvell/db-88f6820-gp/binary.0

@@ -1,16 +0,0 @@
---------
-WARNING:
---------
-This file should contain the bin_hdr generated by the original Marvell
-U-Boot implementation. As this is currently not included in this
-U-Boot version, we have added this placeholder, so that the U-Boot
-image can be generated without errors.
-
-If you have a known to be working bin_hdr for your board, then you
-just need to replace this text file here with the binary header
-and recompile U-Boot.
-
-In a few weeks, mainline U-Boot will get support to generate the
-bin_hdr with the DDR training code itself. By implementing this code
-as SPL U-Boot. Then this file will not be needed any more and will
-get removed.