perf parse-events: Remove unused variable: error

Remove the 'error' variable because it is declared but not used in
parse-events.y or in the generated parse-events.c.

Signed-off-by: Luke Mujica <lukemujica@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20190703222509.109616-2-lukemujica@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>