Documentation: remove mic/index from misc-devices/index.rst

With the recent removal of the misc/mic/ directory, the documentation
build now warns because we forgot about this index file.

Fix that up so that there are no more warnings here.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Sudeep Dutt <sudeep.dutt@intel.com>
Link: https://lore.kernel.org/r/20201103083408.GA2511903@kroah.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>