[js_shared] Remove feature detection for Promise.

MDN says Promise has been widely available across browsers since 2015,
so there should be no need to perform this extra check.

Change-Id: I6a6a69642997f15cd97e5be0cec0974d6dcef5ee
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450262
Reviewed-by: Srujan Gaddam <[email protected]>
Commit-Queue: Mayank Patke <[email protected]>

Author: fishythefish

sdk/lib/_internal/js_shared/lib/js_util_patch.dart

@@ -620,8 +620,7 @@ bool isJavaScriptSimpleObject(value) {
       JS('bool', '# === null', proto);
 }
 
-bool _isJavaScriptPromise(value) =>
-    JS('bool', r'typeof Promise != "undefined" && # instanceof Promise', value);
+bool _isJavaScriptPromise(value) => JS('bool', r'# instanceof Promise', value);
 
 DateTime _dateToDateTime(date) {
   int millisSinceEpoch = JS('int', '#.getTime()', date);