Remove basic_string<unsigned_char> from embind

Only `char`, `wchar`, `char8`, `char16`, and `char32` are valid
specialization for `std::basic_string`:
https://en.cppreference.com/w/cpp/string/basic_string

But libc++ had a base template for `basic_string` that allows any type
to be passed for a long time. It looks there have been several attempts
to remove this but they restored it afterwards due to some complaints,
in chronological order:
llvm/llvm-project@aeecef0
llvm/llvm-project@08a0faf
llvm/llvm-project@e30a148
llvm/llvm-project#66153
llvm/llvm-project#72694

The last one, llvm/llvm-project#72694,
eventually removed it. So `std::basic_string<unsigned_char>` is not
allowed anymore.

Author: aheejin

system/lib/embind/bind.cpp

@@ -149,8 +149,6 @@ EMSCRIPTEN_BINDINGS(builtin) {
   register_float<double>("double");
 
   _embind_register_std_string(TypeID<std::string>::get(), "std::string");
-  _embind_register_std_string(
-    TypeID<std::basic_string<unsigned char>>::get(), "std::basic_string<unsigned char>");
   _embind_register_std_wstring(TypeID<std::wstring>::get(), sizeof(wchar_t), "std::wstring");
   _embind_register_std_wstring(TypeID<std::u16string>::get(), sizeof(char16_t), "std::u16string");
   _embind_register_std_wstring(TypeID<std::u32string>::get(), sizeof(char32_t), "std::u32string");