doc: coding guidelines: remove confusing misra severity column

The misra severity is being treated as relevant to whether a guideline
should be followed or not in the context of the zephyr project.

Remove from the table, the MISRA severity can be looked up in many
public places or in the MISRA coding guielines themselves.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif