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

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 <[email protected]>
Acked-by: Adrian Hunter <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>