[bsp][lpc176x] update scons script.

bsp/lpc176x/SConstruct

@@ -14,7 +14,6 @@ TARGET = 'rtthread-lpc17xx.' + rtconfig.TARGET_EXT
 env = Environment(tools = ['mingw'],
 	AS = rtconfig.AS, ASFLAGS = rtconfig.AFLAGS,
 	CC = rtconfig.CC, CCFLAGS = rtconfig.CFLAGS,
-	CXX = rtconfig.CXX, CXXFLAGS = rtconfig.CXXFLAGS,
 	AR = rtconfig.AR, ARFLAGS = '-rc',
 	LINK = rtconfig.LINK, LINKFLAGS = rtconfig.LFLAGS)