x86/wmi: delete unused wmi_data_lock mutex causing gcc warning

In commit bff431e4 ("ACPI: WMI: Add
ACPI-WMI mapping driver") this mutex was added, but the rest of the
final commit never actually made use of it, resulting in:

 In file included from include/linux/mutex.h:29:0,
                  from include/linux/kernfs.h:13,
                  from include/linux/sysfs.h:15,
                  from include/linux/kobject.h:21,
                  from include/linux/device.h:17,
                  from drivers/platform/x86/wmi.c:35:
 drivers/platform/x86/wmi.c:48:21: warning: ‘wmi_data_lock’ defined but not used [-Wunused-variable]
  static DEFINE_MUTEX(wmi_data_lock);
                      ^

A git grep shows no other instances/references to the wmi_data_lock.
Delete it, assuming that the mutex addition was just a leftover from
an earlier work in progress version of the change, since the original
dates from 2008.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>