[libc++] Remove emscripten handling from exception_fallback.ipp

[sbc100]

Summary

Emscripten doesn't use this file (at least not anymore), it uses
exception_libcxxabi.ipp since _LIBCPPABI_VERSION is defined.

[ldionne]

Thanks for the cleanup. Do you need me to commit this on your behalf? If so, please provide Author Name <email@domain>.

libcxx/src/support/runtime/exception_fallback.ipp

 }
 
 terminate_handler
 get_terminate() _NOEXCEPT
 {
   return __libcpp_atomic_load(&__terminate_handler);
 }
 
-#ifndef __EMSCRIPTEN__ // We provide this in JS
 _LIBCPP_NORETURN
 void
 terminate() _NOEXCEPT
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
     {
 #endif  // _LIBCPP_NO_EXCEPTIONS
         (*get_terminate())();
         // handler should not return
         fprintf(stderr, "terminate_handler unexpectedly returned\n");
         ::abort();
 #ifndef _LIBCPP_NO_EXCEPTIONS
     }
     catch (...)
     {
         // handler should not throw exception
         fprintf(stderr, "terminate_handler unexpectedly threw an exception\n");
         ::abort();
     }
 #endif  // _LIBCPP_NO_EXCEPTIONS
 }
-#endif // !__EMSCRIPTEN__
 
-#if !defined(__EMSCRIPTEN__)
 bool uncaught_exception() _NOEXCEPT { return uncaught_exceptions() > 0; }
 
 int uncaught_exceptions() _NOEXCEPT
 {
 #warning uncaught_exception not yet implemented
   fprintf(stderr, "uncaught_exceptions not yet implemented\n");
   ::abort();
 }
-#endif // !__EMSCRIPTEN__
 
 
 exception::~exception() _NOEXCEPT
 {
 }
 
 const char* exception::what() const _NOEXCEPT
 {