rpmsg: core: Make remove handler for rpmsg driver optional.

Most other bus (for example, SPI, i2c) have the remove handler for
driver optional. Make remove handler for rpmsg driver optional too.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>