Theme.json: Remove template part data from parent theme.

Having the partials set up with area data is causing the template-part block to use that tagName when rendering the template. This causes issues for our header and footer blocks, which each use `header` and `footer` tags themselves.

This removes the other template partials as well, because they're not being used in the child themes.

See WordPress/five-for-the-future#325.

source/wp-content/themes/wporg-parent-2021/theme.json

@@ -995,37 +995,5 @@
 			"title": "No Secondary Content",
 			"postTypes": [ "page", "post" ]
 		}
-	],
-	"templateParts": [
-		{
-			"name": "header",
-			"title": "Header",
-			"area": "header"
-		},
-		{
-			"name": "cover",
-			"title": "Cover",
-			"area": "uncategorized"
-		},
-		{
-			"name": "entry-header",
-			"title": "Post Header",
-			"area": "uncategorized"
-		},
-		{
-			"name": "entry-header-no-meta",
-			"title": "Page Header",
-			"area": "uncategorized"
-		},
-		{
-			"name": "footer-simple",
-			"title": "Footer without Secondary Content",
-			"area": "footer"
-		},
-		{
-			"name": "footer",
-			"title": "Footer",
-			"area": "footer"
-		}
 	]
 }