mp3: removed double cmd==STOP check

cmd has already been checked before, it cannot have changed meanwhile
because it is a local variable.

src/inputPlugins/mp3_plugin.c

@@ -966,9 +966,6 @@ static int mp3Read(mp3DecodeData * data, struct decoder *decoder,
 				}
 
 				data->outputPtr = data->outputBuffer;
-
-				if (cmd == DECODE_COMMAND_STOP)
-					break;
 			}
 		}