support only utf-8 and utf-16 for source code

utf-16 is optionally used by java and msvc. it doesn't hurt to support
it. all other encodings are not used anyway (other than 8-bit
pass-through, but this is a matter for (the also removed) CodecForTr).

Change-Id: Ie6482e187a9a8276918b53e97faeac88a40eb52c
Reviewed-by: hjk <hjk121@nokiamail.com>