It has been added in 6db42a2b,
yet since then none of the necessary create/free_device_capabilities
functions has been implemented, making this API completely useless.

Because of this one can already simplify
avdevice_capabilities_free/create and can already remove the function
pointers at the next major bump; given that the documentation explicitly
states that av_device_capabilities is not to be used by a user, it's
options can already be removed (save for the sentinel).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>