Removal of BLT_lang_is_ime_supported which is always returns true and
is no longer needed.

At one time we only allowed IME entry (Chinese, Japanese, Korean) when the OUTPUT language was any of these. This was because we only used our international font when these languages were selected for output. But since using the international font always, we allowed any language input regardless of the language output. So you can be showing the Blender interface in Spanish but then enter Japanese text.

So this patch simply removes some code that is now cruft. BLT_lang_is_ime_supported() used to tell us if the output language supported IME, but now it always just returns true. In theory it returns false if not compiling with WITH_INPUT_IME defined but then this function is never called in that circumstance anyway.