Clean up a bunch of internal methods and fields that are now unnecessary

The _owner field is unnecessary since it's reachable from _currentElement.

The _lifeCycle field is unnecessary because an internal component should
not even need to exist at all if it's unmounted. It should be dereferenced
internally, and never exposed externally.

The only case where it's important is for batching updates where we
currently avoid calling performUpdateIfNecessary if it's mounted. However,
this function is already only executed "if necessary" so we just make sure
that it's not necessary after unmount by resetting all the pending fields.

Drop ReactOwner.Mixin. This is not used anywhere else. To avoid over-abstraction we should just inline this.

Drop this.props internally. It's only exposed on the public instance.

Drop inaccessible methods that are no longer accessible and the isOwnedBy check
is only used in a method that is not accessible.