refactor: remove Deno.core (#16881)

This commit removes "Deno.core" namespace. It is strictly private API
that has no stability guarantees, we were supposed to remove it long time ago.

Co-authored-by: Yoshiya Hinosawa <stibium121@gmail.com>

cli/tests/unit/text_encoding_test.ts

@@ -264,8 +264,8 @@ Deno.test(function textEncoderShouldCoerceToString() {
 });
 
 Deno.test(function binaryEncode() {
-  // @ts-ignore: Deno.core allowed
-  const ops = Deno.core.ops;
+  // @ts-ignore: Deno[Deno.internal].core allowed
+  const ops = Deno[Deno.internal].core.ops;
   function asBinaryString(bytes: Uint8Array): string {
     return Array.from(bytes).map(
       (v: number) => String.fromCodePoint(v),