tools/img2brec.sh: Delete unused tool

This script was only used on the MX1ADS board (and possibly other MX1
platforms) to program the flash.  As we no longer have any boards for
that SoC, remove this tool.

Fixes: e570aca9 ("mx1ads: remove board support")
Signed-off-by: Tom Rini <trini@konsulko.com>