doc: reset the default sphinx literal block format

doc: reset the default sphinx literal block format

Set the default literal block format for Sphinx docs
to 'none' instead of the implicit default of 'python'.

This means that default :: blocks won't have any
highlighting applied. Explicit highlighting via
the code-block:: directive is already available and
in use in the docs.

Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>