perf scripts python intel-pt-events: Delete unused 'event_attr variable

The 'event_attr' is never used later, the var is ok be deleted.

Additional code simplification is to substitute string slice comparison
with "substring" function. This case no need to know the length specific
words.

Signed-off-by: Alexander Pantyukhin <apantykhin@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20230114130533.2877-1-apantykhin@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>