samples/audio/sof: remove zephyr_interface_library_named(sof_lib)

As this makes no difference, it's misleading so let's remove it to stop
being misled. I don't know why initial commit efa794db added it but
the build directory is strictly identical with or without this line.

The SOF code is included in the build thanks to CONFIG_SOF in prj.conf,
not this.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>