RDMA/core: Remove the duplicate header file

The header file rdma_core.h is duplicate, so let's remove it.

Fixes: 622db5b6

 ("RDMA/core: Add trace points to follow MR allocation")
Link: https://lore.kernel.org/r/20200310091656.249696-1-leon@kernel.org
Signed-off-by: Zhu Yanjun <yanjunz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>