LibThread: Remove LOCKER() macro, as it adds no value

The LOCKER() macro appears to have been added to LibThread as a
userspace analog to the previous LOCKER() macro that existed in
the kernel. The kernel version used the macro to inject __FILE__ and
__LINE__ number into the lock acquisition for debugging. However
AK::SourceLocation was used to remove the need for the macro. So
the kernel version no longer exists. The LOCKER() in LibThread doesn't
appear to actually need to be a macro, using the type directly works
fine, and arguably is more readable as it removes an unnecessary
level of indirection.

Userland/Libraries/LibThread/BackgroundAction.h

@@ -53,7 +53,7 @@ private:
         , m_action(move(action))
         , m_on_complete(move(on_complete))
     {
-        LOCKER(all_actions().lock());
+        Locker locker(all_actions().lock());
 
         all_actions().resource().enqueue([this] {
             m_result = m_action();