perf script: Stop using pevent directly

perf script: Stop using pevent directly

We can get all that is needed using just event_format, that is available
via evsel->tp_format now.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-2hsr1686epa9f0vx4yg7z2zj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>