remove == operator override

https://github.com/marcelgarus/marquee/blob/df656fb04b05b269409b1c755de23906642dd430/lib/marquee.dart#L517-L532

these lines are pointless. first of all, what is the use case for such override?
secondly this won't work properly until you override [hashCode](https://api.flutter.dev/flutter/dart-core/Object/hashCode.html) as well

Open lib/marquee.dart at lines 517-532 and inspect the == operator override in context. Remove the override as requested, then verify that the package remains valid and that the widget's existing behavior is unchanged.

Written by the indexing model from the issue text.