Avoid override of variable throttle for turbo.

It is not needed to set `throttle` on neither case. There is actually a
bug when using turbo/speedup and closing the emulator. The test case
is the following:

#627

On Windows:

1. use opengl;
2. load GBA game;
3. set throttle to `100%`;
4. set turbo throttle to `200%`;
5. enable turbo on menu;
6. save game;
7. close emulator && open emulator;
8. load GBA game;

==>

throttle is `200%`.

This is definitely not expected.

src/gb/GB.cpp

@@ -4953,7 +4953,7 @@ void gbEmulate(int ticksToStop)
                     if (turbo_button_pressed) {
                         if (!speedup_throttle_set && throttle != speedup_throttle) {
                             last_throttle = throttle;
-                            throttle = speedup_throttle;
+                            //throttle = speedup_throttle;
                             soundSetThrottle(speedup_throttle);
                             speedup_throttle_set = true;
                         }
@@ -4970,7 +4970,7 @@ void gbEmulate(int ticksToStop)
                             framesToSkip = speedup_frame_skip;
                     }
                     else if (speedup_throttle_set) {
-                        throttle = last_throttle;
+                        //throttle = last_throttle;
                         soundSetThrottle(last_throttle);
 
                         speedup_throttle_set = false;

src/gba/GBA.cpp

@@ -3799,7 +3799,7 @@ void CPULoop(int ticks)
                     if (turbo_button_pressed) {
                         if (!speedup_throttle_set && throttle != speedup_throttle) {
                             last_throttle = throttle;
-                            throttle = speedup_throttle;
+                            //throttle = speedup_throttle;
                             soundSetThrottle(speedup_throttle);
                             speedup_throttle_set = true;
                         }
@@ -3816,7 +3816,7 @@ void CPULoop(int ticks)
                             framesToSkip = speedup_frame_skip;
                     }
                     else if (speedup_throttle_set) {
-                        throttle = last_throttle;
+                        //throttle = last_throttle;
                         soundSetThrottle(last_throttle);
 
                         speedup_throttle_set = false;