allow KaxChapterTranslate* to be written again

allow KaxChapterTranslate* to be written again

Merging PR #33 was supposed to only prevent writing of
KaxFileUsed{Start,End}Time. Unfortunately git make a mess of it in
5ba4db4449507e32ff78bbec71c58a6d6065c419 where KaxSemantic.cpp was
correct but the change to KaxSemantic.h was written at the wrong
position, accidentally disabling writing for KaxChapterTranslate*

I didn't realize this, fixed the situation for KaxFileUsed* in
2be299cd6617427fe5c89f1894a9d91fd9e2ed08 and made the situation worse
for KaxChapterTranslate* in b21e0d395d9f0e94e94008a5d685cead6fa45cac.

This commit restores the functionality for KaxChapterTranslate* while
still leaving KaxFileUsed* disabled.