net: lib: coap_client: Remove unnecessary atomic variable

In receiving thread, continuing the loops is based on
has_ongoing_exchanges() so it does not need atomic
coap_client_recv_active variable.

When idling, it wakes from semaphore. But there was potential
deadlock when coap_client_schedule_poll() would not signal the
semaphore, if atomic variable was already showing that it runs.
Removing the atomic variable removes this deadlock.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>