AP_HAL_Linux: PWM_Sysfs: remove unneeded free() in ~PWM_Sysfs_Base

_export_path and _duty_path will have been already long time gone by the
time dtors kick in.

Probably better to use OwnPtr around those. But it's better to be done
in a separate PR.

libraries/AP_HAL_Linux/PWM_Sysfs.cpp

@@ -46,10 +46,8 @@ PWM_Sysfs_Base::~PWM_Sysfs_Base()
 {
     ::close(_duty_cycle_fd);
 
-    free(_export_path);
     free(_polarity_path);
     free(_enable_path);
-    free(_duty_path);
     free(_period_path);
 }