Removing etc1 from build for javascript platform.

Based on inputs from reduz
1) Made etc1 optional driver and default yes.
2) Reverted the hack in rg_etc1.cpp
3) Disabled etc1 for javascript plaform.

platform/javascript/detect.py

@@ -36,6 +36,7 @@ def get_flags():
 		('squish', 'no'),
 		('speex', 'no'),
 		('old_scenes', 'no'),
+		('etc1', 'no'),
 #		('default_gui_theme', 'no'),
 
 		#('builtin_zlib', 'no'),