ofp-errors: Remove OFPERR_NXFMFC_GROUPS_NOT_SUPPORTED

ofp-errors: Remove OFPERR_NXFMFC_GROUPS_NOT_SUPPORTED

OFPERR_NXFMFC_GROUPS_NOT_SUPPORTED is currently only used in paths which
are part of a non-NX extension portions of the Open Flow 1.1+
implementation.

After recent discussion it has been decided to attempt to only use
standardised, albeit lest-specify, errors unless errors arise from use of
an NX extension.

With the above in mind it seems appropriate to:
* Use OFPERR_OFPFMFC_UNKNOWN in place of OFPERR_NXFMFC_GROUPS_NOT_SUPPORTED.
* Remove OFPERR_NXFMFC_GROUPS_NOT_SUPPORTED as it is no longer used.

An unfortunate side-effect of this change is that the error for
the case in question is now less-specific.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>