removed declared but undefined LanguageTagImpl::convertFromRtlLocale()

Change-Id: I928fb77181991948216bb18cdfcea0816218a2b9
(cherry picked from commit 5d34cd53)

i18nlangtag/source/languagetag/languagetag.cxx

@@ -301,8 +301,6 @@ private:
     void                convertLangToLocale();
     void                convertLangToBcp47();
 
-    void                convertFromRtlLocale();
-
     /** @return whether BCP 47 language tag string was changed. */
     bool                canonicalize();