log: websocket: Remove debug printing

The debug printing feature seems to be broken by design - not only in a
default configuration it would trigger endless logging loop (as printk
is piped through logging subsys), it also doesn't work well with
deferred logging (additional artifacts in the output).
Since similar effect can be achieved by enabling any other logger
backend, which should receive the same data as the websocket one, simply
remove it.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>