sessions: remove sessions attr del function def

The sessions attribute functionality was never approved as part of the MPI-4 sessions feature.
Remove a vestige of this from Open MPI.

Related to open-mpi#10388
Related to open-mpi#9097

Signed-off-by: Howard Pritchard <[email protected]>
(cherry picked from commit c488c60)

Author: hppritcha

ompi/include/mpi.h.in

@@ -513,7 +513,6 @@ typedef int (MPI_Type_delete_attr_function)(MPI_Datatype, int,
 typedef int (MPI_Win_copy_attr_function)(MPI_Win, int, void *,
                                            void *, void *, int *);
 typedef int (MPI_Win_delete_attr_function)(MPI_Win, int, void *, void *);
-typedef int (MPI_Session_delete_attr_function)(MPI_Session, int, void *, void *);
 typedef int (MPI_Grequest_query_function)(void *, MPI_Status *);
 typedef int (MPI_Grequest_free_function)(void *);
 typedef int (MPI_Grequest_cancel_function)(void *, int);