GBA: Disable zombie mode for GB audio channels

Caused sound issues in some games (i.e M&L Superstar saga)

Core/GBA/APU/GbaEnvelope.h

@@ -41,41 +41,9 @@ class GbaEnvelope
 		if((value & 0xF8) == 0) {
 			state.Enabled = false;
 		} else {
-			//This implementation of the Zombie mode behavior differs from the description
-			//found here: https://gbdev.gg8.se/wiki/articles/Gameboy_sound_hardware
-			//Instead, it's based on the behavior of the channel_1_nrx2_glitch test and
-			//and SameBoy's implementation of the glitch
-			bool preventIncrement = false;
-			if(raiseVolume != state.EnvRaiseVolume) {
-				if(raiseVolume) {
-					if(!state.EnvStopped && state.EnvPeriod == 0) {
-						state.Volume ^= 0x0F;
-					} else {
-						state.Volume = 14 - state.Volume;
-					}
-					preventIncrement = true;
-				} else {
-					//"If the mode was changed (add to subtract or subtract to add), volume is set to 16 - volume."
-					state.Volume = 16 - state.Volume;
-				}
-
-				//"Only the low 4 bits of volume are kept"
-				state.Volume &= 0xF;
-			}
-
-			if(!state.EnvStopped && !preventIncrement) {
-				if(state.EnvPeriod == 0 && (period || raiseVolume)) {
-					if(raiseVolume) {
-						//"If the old envelope period was zero and the envelope is still doing automatic updates, volume is incremented by 1"
-						state.Volume++;
-					} else {
-						state.Volume--;
-					}
-
-					//"Only the low 4 bits of volume are kept"
-					state.Volume &= 0xF;
-				}
-			}
+			//No zombie mode for GBA? (or maybe it behaves differently.)
+			//Using the GB implementation of zombie mode causes sound issues in some games
+			//e.g popping can be heard in M&L Superstar Saga's menu at the start of the game
 		}
 
 		state.EnvPeriod = period;