doc: bootdelay: drop explanation about CONFIG_BOOTDELAY from README

The same information now exists in common/Kconfig.  Do not duplicate
documentation from the point of view of maintainability.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>