SUNRPC: Remove redundant calls to RPC_IS_QUEUED()

The RPC task wakeup calls all check for RPC_IS_QUEUED() before taking any
locks. In addition, rpc_exit() already calls rpc_wake_up_queued_task().

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>