Editor Settings: remove duplicate "Replace Image" button when editing the Featured Image

[ciampo]

Steps to reproduce the issue

1. On a WordPress.com site (simple or atomic), create a new post
2. Open the settings sidebar (by clicking the cog icon in the top right of the editor)
3. Select the "Post" tab and scroll to the "Featured Image" section
4. Notice the "Select Image" button. When clicking this button, a dropdown appears showing media source selection.
5. Add a Featured Image
6. One the image has uploaded, notice the buttons UI: "Replace Image", "Select Image", "Remove featured image"

What I expected

When a featured image is selected:

• The original "Replace Image" button (coming from core gutenberg) should not be visible
• The "Select Image" button (from Jetpack plugin) should be visible, but its label text should say "Replace Image"
• There should be some vertical space before the "Remove featured image" link/button

What happened instead

The "Replace Image" button (from core Gutenberg) and the "Select Image" button are both shown to the user, and there are some misalignments in the buttons UI

Screenshots

Context

This issue was part of a larger exploration (see this GitHub issue) about the discrepancies between .org and .com sites when editing a post's featured image.

A good starting point when working on this issue is:

jetpack/projects/plugins/jetpack/extensions/shared/external-media/index.js

Lines 53 to 60 in e13487d

	if ( isAllowedBlock( name, render ) || isFeaturedImage( props ) ) {
	const { allowedTypes, gallery = false, value = [] } = props;
	// Only replace button for components that expect images, except existing galleries.
	if ( allowedTypes.indexOf( 'image' ) > -1 && ! ( gallery && value.length > 0 ) ) {
	render = button => <MediaButton { ...button } mediaProps={ props } />;
	}
	}