perf tools: Rename perf_event::bpf_event to perf_event::bpf

Just like all the other meta events, that extra _event suffix is just
redundant, ditch it.

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-505qwpaizq1k0t6pk13v1ibd@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>