RDMA: Remove Sean's and Hal's emails from MAINTAINER file

RDMA subsystem has one active maintainer who can apply
patches - Doug Ledford, but the RDMA entries in MAINTAINER file
are not stating it. The following patch removes Sean's and Hal's
emails from the maintainers list.

It will allow clean get_maintaner.pl output which is needed for people
outside of our community and various semi-automatic tools.

Cc: Sean Hefty <sean.hefty@intel.com>
Cc: Hal Rosenstock <hal@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>