icu: Drop binconfig support (icu-config)

Upstream have indicated icu-config is deprecated and they don't intend
people to use it any more. Drop our code for it to flush out users and
clean up our recipe a bit. pkg-config is a much better alternative.

(https://github.com/unicode-org/icu/pull/2150

)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>