build system: no longer prompt for PLATFORM_LINUX option

With the new "select PLATFORM_LINUX" mechanism
(commit e3b1a1fd28558f7a1b3c0ec33313bedb675be8a1), the PLATFORM_LINUX
option alone no longer has any purpose of changing program behavior or
affecting compiled code. So there is no longer need to prompt user of
this config question.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed