tools/perf/build: Simplify the libelf logic

Ulrich Drepper and Namhyung Kim reported that the libelf logic in
config/Makefile is duplicated in part.

Remove the duplication, and also remove the now unused FLAGS_LIBELF
variable.

Reported-by: Ulrich Drepper <drepper@gmail.com>
Reported-by: Namhyung Kim <namhyung@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>