Removing unnecessary div wrapper on slides

Currently, react-slick wraps each slide with div, which deviates from the slick-carousel implementation.
So we will remove unnecessary div after fixing other pending issues in this library.
Hopefully, we will make a release with this fix in a month or two.

This would be breaking change.
When writing CSS be aware that div wrapper on the slide will be removed in future.

Using this issue for tracking progress and discussions related to this change.