bus: mhi: host: Remove mhi_poll() API

mhi_poll() API is not used within the MHI stack and also not by any client
drivers in mainline. So let's remove it until any consumer is available.

Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>