[Dot Shorthands] `ReplaceWithVar` fix - Remove unnecessary `.new` in fix

[kallentu]

Follow up from https://dart-review.googlesource.com/c/sdk/+/441320

We insert the type on the constructor in the ReplaceWithVar fix and we should tweak it to handle unnamed constructors better.
For example:

class E {}
E f() {
  ^E e = .new();
  return e;
}

Currently, the fix for the above is:

class E {}
E f() {
  var e = E.new(); // E() instead
  return e;
}

We'd like E(); instead.