perf cs-etm: Use existing decoder instead of resetting it

When dumping trace, the decoder is continually deleted and recreated to
decode each buffer. To support both formatted and unformatted trace in
a later commit, the decoder will be configured in advance.

This commit removes the deletion of the decoder and allows the
formatted/unformatted setting to persist.

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: James Clark <james.clark@arm.com>