Remove useless STACKALIGN macro.

It only ever did anything on 32-bit OS X.

Anyway, it wasn't even on the right functions, and these days
ABI_PushRegistersAndAdjustStack should handle maintaining the ABI
correctly.

Source/Core/VideoBackends/Software/SWCommandProcessor.cpp

@@ -174,7 +174,7 @@ void RegisterMMIO(MMIO::Mapping* mmio, u32 base)
 	);
 }
 
-void STACKALIGN GatherPipeBursted()
+void GatherPipeBursted()
 {
 	if (cpreg.ctrl.GPLinkEnable)
 	{