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/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