Removed glfwGetScrollOffset.

Scroll events do not represent an absolute state, but rather an interpretation
of a relative change in state, like character input.  So, like character input,
there is no sane 'current state' to return.  The here removed solution, that of
accumulating an offset since the last call to event processing, is at best
mildly confusing.  If a user wishes to implement this solution, it is better for
it to be explicit in client code than implicit in GLFW calls.

src/window.c

@@ -48,22 +48,6 @@ static int Max(int a, int b)
 }
 
 
-//========================================================================
-// Clear scroll offsets for all windows
-//========================================================================
-
-static void clearScrollOffsets(void)
-{
-    _GLFWwindow* window;
-
-    for (window = _glfw.windowListHead;  window;  window = window->next)
-    {
-        window->scrollX = 0;
-        window->scrollY = 0;
-    }
-}
-
-
 //////////////////////////////////////////////////////////////////////////
 //////                         GLFW event API                       //////
 //////////////////////////////////////////////////////////////////////////
@@ -746,8 +730,6 @@ GLFWAPI void glfwPollEvents(void)
         return;
     }
 
-    clearScrollOffsets();
-
     _glfwPlatformPollEvents();
 }
 
@@ -759,8 +741,6 @@ GLFWAPI void glfwWaitEvents(void)
         return;
     }
 
-    clearScrollOffsets();
-
     _glfwPlatformWaitEvents();
 }