Revert "Enable layers in non-tile modes."

We need to use proper documented behaviour when coding the
core, therefore this should be reverted until a proper
solution is done.

This reverts commit ca3b63d64c.

src/gba/GBA.cpp

@@ -577,7 +577,6 @@ extern uint32_t line3[240];
 
 void CPUUpdateRenderBuffers(bool force)
 {
-    layerEnable = layerSettings & DISPCNT;
     if (!(layerEnable & 0x0100) || force) {
         CLEAR_ARRAY(line0);
     }