Ifdef out network code for M&P

M&P contains network code despite M&P not being a Steam/GOG release (as
Steam/GOG releases are full releases of the game, not
custom-levels-only releases).

While unlocking achievements is already ifdef'd out in M&P, let's remove
the network code entirely to make sure people can't do other shenanigans
with M&P builds, and also to have a smaller binary size.

desktop_version/src/SteamNetwork.c

@@ -1,3 +1,7 @@
+#include "MakeAndPlay.h"
+
+#ifndef MAKEANDPLAY
+
 #include <stdio.h>
 #include <stdint.h>
 #include <SDL.h>
@@ -218,3 +222,5 @@ void STEAM_setAchievementProgress(const char *name, int32_t stat)
 		SteamAPI_ISteamUserStats_StoreStats(steamUserStats);
 	}
 }
+
+#endif /* MAKEANDPLAY */