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.cpp

@@ -17,7 +17,7 @@ static intptr_t background_thread_func()
     while (true) {
         Function<void()> work_item;
         {
-            LOCKER(s_all_actions->lock());
+            LibThread::Locker locker(s_all_actions->lock());
 
             if (!s_all_actions->resource().is_empty())
                 work_item = s_all_actions->resource().dequeue();