Drop check for libc++ >= 15

We have required LLVM 15 for a while.

- Don't mark workerd binary as no-arm64, which caused problems for
  downstream where it still needs to be built.

src/workerd/jsg/iterator.h

@@ -160,23 +160,13 @@ class GeneratorImpl {
   friend Generator;
 };
 
-#if _LIBCPP_VERSION >= 15000
 template <class Func, class T>
 concept GeneratorCallback = std::invocable<Func, Lock&, T, GeneratorContext<T>&> &&
     std::same_as<void, std::invoke_result_t<Func, Lock&, T, GeneratorContext<T>&>>;
 
 template <class Func, class T>
 concept AsyncGeneratorCallback = std::invocable<Func, Lock&, T, GeneratorContext<T>&> &&
     std::same_as<Promise<void>, std::invoke_result_t<Func, Lock&, T, GeneratorContext<T>&>>;
-#else
-// These don't work in libc++ 11. I haven't tried 12-14, but they do work in 15. So we'll just
-// skip the extra type checking if we don't have libc++ 15 or newer.
-template <class Func, class T>
-concept GeneratorCallback = true;
-
-template <class Func, class T>
-concept AsyncGeneratorCallback = true;
-#endif
 
 template <typename T>
 class Generator final {

src/workerd/server/BUILD.bazel

@@ -45,7 +45,6 @@ wd_cc_binary(
     name = "workerd",
     srcs = ["workerd.c++"],
     malloc = ":malloc",
-    tags = ["no-arm64"],
     visibility = ["//visibility:public"],
     deps = [
         ":server",