mesh: Remove GTKdata and IGTKdata from Mesh Peering Confirm/Close

These optional fields are supposed to be included in the Authenticated
Mesh Peering Exchange element only in Mesh Peering Open frames.
Previously, these were incorrectly included in Mesh Peering
Confirm/Close frames and also required to be present in all these
frames.

While this commit changes the receive processing to ignore the
unexpected extra fields, it should be noted that the previous
implementation required the fields to be present and as such, the fixed
implementation is not compatible with it for secure mesh.

Signed-off-by: Jouni Malinen <j@w1.fi>