Remove the need for a root container class

[ZeeCoder]

Currently in order for react-container-query to work, the root element wrapped with the helper components must have the container class on it.

Instead, just the act of creating a Container instance should be enough to "mark" that element as the container element.

An arbitrary class's presence should not be necessary on top of that.

[ZeeCoder]

Turns out, this is exactly how it's been working already.