xprtrdma: Remove unused function declaration rpcrdma_bc_post_recv()

rpcrdma_bc_post_recv() is never implemented since introduction in
commit f531a5db ("xprtrdma: Pre-allocate backward rpc_rqst and send/receive buffers").

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>