src: remove unreachable UNREACHABLE

All paths leading up to this UNREACHABLE either return or abort. (If
they did not, the compiler would at least emit a warning.)

PR-URL: #46281
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

src/string_bytes.cc

@@ -684,11 +684,8 @@ MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
     }
 
     default:
-      CHECK(0 && "unknown encoding");
-      break;
+      UNREACHABLE("unknown encoding");
   }
-
-  UNREACHABLE();
 }