cs-etm: removing unecessary structure field

Function cs_etm__sample() is called only from cs_etm__run_decoder() where
cs_etm_queue::have_sample is set to 'true'.  As such checking the value of
the variable again in cs_etm__sample() is not needed.

Since the variable isn't used anywhere else, also removing it from the
structure definition.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>