net/rds: Remove unused function declarations

Commit 39de8281 ("RDS: Main header file") declared but never implemented
rds_trans_init() and rds_trans_exit(), remove it.
Commit d37c9359 ("RDS: Move loop-only function to loop.c") removed the
implementation rds_message_inc_free() but not the declaration.

Since commit 55b7ed0b ("RDS: Common RDMA transport code")
rds_rdma_conn_connect() is never implemented and used.
rds_tcp_map_seq() is never implemented and used since
commit 70041088 ("RDS: Add TCP transport to RDS").

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>