x86/platform/intel-mid: Remove unused leftovers (msic_audio)

Commit 05f4434b ("ASoC: Intel: remove mfld_machine") removed the
driver, no need to have support files for it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>