perf tools: Remove unused functions from debugfs object

Following debugfs object functions are not referenced
within the code:

  int debugfs_valid_entry(const char *path);
  int debugfs_umount(void);
  int debugfs_write(const char *entry, const char *value);
  int debugfs_read(const char *entry, char *buffer, size_t size);
  void debugfs_force_cleanup(void);
  int debugfs_make_path(const char *element, char *buffer, int size);

Removing them.

Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1327674868-10486-3-git-send-email-jolsa@redhat.com
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>