`onLoad` content is added to undo stack

Despite the intent of this code to prevent initial `onLoad` content from being added to the undo stack, it does get added and is undoable:

https://github.com/Automattic/isolated-block-editor/blob/82b69eda507554c76b4afecc1f75987955c22e36/src/components/block-editor-contents/index.js#L75-L86

(This behavior was fixed for undo in collab mode #68, but is not yet fixed for the non-collab case.)

## Steps to reproduce

For convenience, here's a branch with a Storybook story for `onLoad` and a failing UI integration test: https://github.com/Automattic/isolated-block-editor/tree/test/onload-undo

1. Load an iso-editor with some `onLoad` content.
2. Once the initial content loads, the Undo button is active.
3. When you trigger an undo, you can wipe out the initial content.

### Expected behavior

The initial `onLoad` content is not undoable.

Start at src/components/block-editor-contents/index.js lines 75-86 and use the test/onload-undo branch's Storybook story and failing UI integration test to reproduce the behavior. Done means initial onLoad content does not activate the Undo button and cannot be removed by undo.

Written by the indexing model from the issue text.