Drop the .json suffix for events directory in the mapfile.csv.

Now that we have separate JSON files for each topic in a CPU (eg: see
tools/perf/pmu-events/arch/powerpc/power8/*.json) the .json suffix in
the mapfile is misleading and redundant.
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Anton Blanchard <anton@au1.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20170802174617.GA32545@us.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>