remove linejoin shader

The previous commit switched to using triangles for drawing round joins.

src/mbgl/geometry/elements_buffer.cpp

@@ -14,8 +14,3 @@ void LineElementsBuffer::add(element_type a, element_type b) {
     elements[0] = a;
     elements[1] = b;
 }
-
-void PointElementsBuffer::add(element_type a) {
-    uint16_t *data = static_cast<element_type *>(addElement());
-    data[0] = a;
-}