Eliminate obsolete call to onChange when RichText componentWillUnmount

[westonruter]

Fixes #6134.

When wp.hooks.addFilter( 'blocks.BlockEdit'… ) is used to extend blocks which contain the RichEdit component, this will cause the component to be unmounted and with that onChange will be called:

gutenberg/blocks/rich-text/index.js

Lines 702 to 704 in 004cb8b

	componentWillUnmount() {
	this.onChange();
	}

This results in the editor being put into a dirty state upon initialization, even if no changes have been made by the user. This bit of code actually appears to be a remnant of the past, @iseulde in #6134 (comment):

Everything should be synced whenever there is a change in the editor. At first sight I think it is fine to remove. It seems to be a remnant from when we only synced the value on blur.

But @youknowriad adds:

I agree with this statement, but I'd test carefully the different usecases, of splitting/merging/switching etc... :) fast typing + splitting

I'm not sure how best to go about testing this change, but I wanted to open a PR at least to get the fix off the ground.

[gziolo]

I like this change. Hopefully, it doesn't introduce any regressions :)

[youknowriad]

LGTM 👍 No regression found. If we encounter one, it will be an excuse to add an E2E test :)