libstdc++: Do not assume existence of char8_t codecvt facet

It is not required that codecvt<char8_t, char, mbstate_t> facet be
supported by the locale, nor is it added as part of the default locale.
This can lead to dangerous behaviour when static_cast.

libstdc++-v3/ChangeLog:

	* include/bits/locale_classes.tcc: Remove check for
	codecvt<char8_t, char, mbstate_t> facet.

libstdc++-v3/include/bits/locale_classes.tcc

@@ -129,9 +129,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _GLIBCXX_STD_FACET(time_put<wchar_t>);
       _GLIBCXX_STD_FACET(messages<wchar_t>);
 #endif
-#ifdef _GLIBCXX_USE_CHAR8_T
-      _GLIBCXX_STD_FACET(codecvt<char8_t, char, mbstate_t>);
-#endif
 #if __cplusplus >= 201103L
       _GLIBCXX_STD_FACET(codecvt<char16_t, char, mbstate_t>);
       _GLIBCXX_STD_FACET(codecvt<char32_t, char, mbstate_t>);