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();

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();

Userland/Libraries/LibThread/Lock.h

@@ -70,8 +70,6 @@ inline void Lock::unlock()
         --m_level;
 }
 
-#    define LOCKER(lock) LibThread::Locker locker(lock)
-
 template<typename T>
 class Lockable {
 public:
@@ -92,7 +90,7 @@ public:
 
     T lock_and_copy()
     {
-        LOCKER(m_lock);
+        Locker locker(m_lock);
         return m_resource;
     }
 
@@ -113,8 +111,12 @@ public:
     ~Lock() { }
 };
 
+class Locker {
+public:
+    explicit Locker(Lock&) { }
+    ~Locker() { }
+};
+
 }
 
-#    define LOCKER(x)
-
 #endif