Fix: remove formats from document outline headings

[jorgefilipecosta]

Description

After #8204 was merged, we started using the heading content as the outline item.
So if the heading contained custom formats, they were shown in the document outline. Showing the formats in the outline may break the formats as discovered by @mcsf.

This PR makes sure we show the plain text in the hading without any formats.

Props to @mcsf for finding the issue and proposing a solution.

How has this been tested?

In the code editor I pasted:

<!-- wp:heading -->
<h2>aaaa</h2>
<!-- /wp:heading -->

<!-- wp:heading {"level":3} -->
<h3><span style="line-height: 100px; color: red">bbbb</span></h3>
<!-- /wp:heading -->

<!-- wp:heading -->
<h2>cccccc</h2>
<!-- /wp:heading -->

I checked that the document inspector shown matches the "After" image.

Screenshots

Before:

After:

[ellatrix]

Look good.