net/mlx5: DR, Remove protocol-specific flex_parser_3 definitions

Remove MPLS specific fields from flex parser 3 layout.
Flex parser can be used for multiple protocols and should
not be hardcoded to a specific type.

Signed-off-by: Muhammad Sammar <muhammads@nvidia.com>
Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>