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 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.

Change-Id: Ia4131151b7efb014716c5a0666f940384975ea42
Signed-off-by: Chris Lew <clew@codeaurora.org>