[core] add polyfill for codecvt for STLs that don't have it yet

platform/default/utf.cpp

@@ -1,25 +1,13 @@
 #include <mbgl/util/utf.hpp>
 
-#include <memory>
 #include <locale>
-
-// GCC 4.9 compatibility
-#if !defined(__GNUC__) || __GNUC__ >= 5
 #include <codecvt>
-#else
-#include <boost/locale/encoding_utf.hpp>
-#endif
 
 namespace mbgl {
 namespace util {
 
-std::u16string utf8_to_utf16::convert(std::string const& utf8) {
-#if !defined(__GNUC__) || __GNUC__ >= 5
-    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> converter;
-    return converter.from_bytes(utf8);
-#else
-    return boost::locale::conv::utf_to_utf<char16_t>(utf8.c_str(), utf8.c_str() + utf8.size());
-#endif
+std::u16string utf8_to_utf16::convert(const std::string& utf8) {
+    return std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t>().from_bytes(utf8);
 }
 
 } // namespace util