RDMA/mlx5: Fix flow creation on representors

The intention of the flow_is_supported was to disable the entire tree and
methods that allow raw flow creation, but the grammar syntax has this
disable the entire UVERBS_FLOW object. Since the method requires a
MLX5_IB_OBJECT_FLOW_MATCHER there is no need to do anything, as it is
automatically disabled when matchers are disabled.

This restores the ability to create flow steering rules on representors
via regular verbs.

Fixes: a1462351 ("RDMA/mlx5: Fail early if user tries to create flows on IB representors")
Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>