cpupower tools: Fix minor warnings

Fix minor warnings reported with GCC 4.6:
* The sysfs_write_file function is unused - remove it.
* The pr_mon_len in the print_header function is unsed - remove it.

Signed-off-by: Palmer Cox <p@lmercox.com>
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>