drm/vc4: hdmi: Remove unused struct

Commit 91e99e11 ("drm/vc4: hdmi: Register HDMI codec") removed the
references to the vc4_hdmi_audio_component_drv structure, but not the
structure itself resulting in a warning. Remove it.

Fixes: 91e99e11 ("drm/vc4: hdmi: Register HDMI codec")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210707093632.1468127-2-maxime@cerno.tech