omap: remove L4_MUX_HACK

This was introduced apparently to overcome a limitation on the number of
cpu_register_io_memory() calls.  477b24ef91175 (July 2008) removed use
of the hack, but retained the code.  This patch removes the code as well.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andrzej Zaborowski <balrogg@gmail.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
