mux: remove the Kconfig question for the subsystem

The MULTIPLEXER question in the Kconfig might be confusing and is
of dubious value. Remove it. This makes consumers responsible for
selecting MULTIPLEXER, which they already do.

Signed-off-by: Peter Rosin <peda@axentia.se>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>