perf scripts python: exported-sql-viewer.py: Remove no selection error

If no selection is made on the 'Selected branches' dialog, then the
output is the same as the 'All branches' report. That is not really an
error, and is not desirable for future reports, so remove it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>