kernel-shark: Remove a duplicate error message

If the capture process finishes with an error the _captureError() slot
will print an error message. We do not need to print identical message
in _captureFinished().

Link: http://lore.kernel.org/linux-trace-devel/20190709155650.2345-6-y.karadz@gmail.com

Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>