scripts: kernel-doc: get rid of unused output formats

Since there isn't any docbook code anymore upstream,
we can get rid of several output formats:

- docbook/xml, html, html5 and list formats were used by
  the old build system;
- As ReST is text, there's not much sense on outputting
  on a different text format.

After this patch, only man and rst output formats are
supported.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>