rpmsg: glink: Don't send pending rx_done during remove
Attempting to transmit rx_done messages after the GLINK instance is
being torn down will cause use after free and memory leaks. So cancel
the intent_work and free up the pending intents.
With this there are no concurrent accessors of the channel left during
qcom_glink_native_remove() and there is therefor no need to hold the
spinlock during this operation - which would prohibit the use of
cancel_work_sync() in the release function. So remove this.