kconfig: drop the ""trying to assign nonexistent symbol" warning

They really stand out now that make *config is less chatty - and
they are generally ignored - so drop them.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Adrian Bunk <bunk@kernel.org>