perf tool: Rename perf_tool::bpf_event to bpf

No need for that _event suffix, do just like all the other meta event
handlers and suppress that suffix.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lkml.kernel.org/n/tip-03spzxtqafbabbbmnm7y4xfx@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>