perf tests wp: Remove unused functions on s390

Fixing these build problems:

  tests/wp.c:24:12: error: 'wp_read' defined but not used [-Werror=unused-function]
   static int wp_read(int fd, long long *count, int size)
              ^
  tests/wp.c:35:13: error: 'get__perf_event_attr' defined but not used [-Werror=unused-function]
   static void get__perf_event_attr(struct perf_event_attr *attr, int wp_type,
               ^
    CC      /tmp/build/perf/util/print_binary.o

Fixes: e47c6eca ("perf test: Convert watch point tests to test cases.")
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: Daniel Latypov <dlatypov@google.com>
Cc: David Gow <davidgow@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sohaib Mohamed <sohaib.amhmd@gmail.com>
Cc: Stephane Eranian <eranian@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>