perf tools: Remove unused trace_find_next_event()

trace_find_next_event() was buggy and pretty much a useless helper. As
there are no more users, just remove it.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20191017210636.224045576@goodmis.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>