Remove unused isCompositeComponentElement re-export

[aweary]

This method is being deprecated soon facebook/react#10096

We don't actually use it at all internally, so this change just removes the unused import/export.

The rewrite doesn't include this import/export so this may be a moot PR, but I'm not sure if we intend to make any more minor releases before it's accepted.

[ljharb]

Removing an export from any file is semver-major. Instead, let's include this change in the rewrite branch, so that the rewrite can be v3.

[aweary]

Semver only dictates major releases for breaking changes to the public API, which should be well defined.

For this system to work, you first need to declare a public API. This may consist of documentation or be enforced by the code itself. Regardless, it is important that this API be clear and precise.

Enzyme has a pretty clear public API defined by the documentation, so I would argue this isn't a breaking change since react-compat exports are not defined as part of the public API (and never should be).

[aweary]

Either way, we can close this if we want to consider it semver major since the rewrite branch doesn't contain this import/export anymore.

[ljharb]

Our public API is defined as "everything that can possibly be reachable or observed", regardless of what's in documentation, which is also the way I think all projects should define it.

[aweary]

I think that's an ill-defined way to describe a public API, but if that's the standard for Enzyme so be it. In that case, this PR is unnecessary since the same change will be made in #1007