tools/perf/build: Simplify the libelf logic

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 <[email protected]>
Reported-by: Namhyung Kim <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>