rpmsg: glink: Remove channel decouple from rpdev release

If a channel is being rapidly restarting and the kobj release worker is
busy, there is a chance the rpdev_release function will run after the
channel struct itself has been released.

There should not be a need to decouple the channel from rpdev in the
rpdev release since that should only happen from the close commands.

Signed-off-by: Chris Lew <clew@codeaurora.org>
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/1596086296-28529-6-git-send-email-deesin@codeaurora.org