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>

Documentation/misc-devices/index.rst

@@ -24,7 +24,6 @@ fit into other categories.
    isl29003
    lis3lv02d
    max6875
-   mic/index
    pci-endpoint-test
    spear-pcie-gadget
    uacce