Issue #3414978: Handle brief page title flicker on Firefox

Doesn't happen in Chrome, but before the original issue was addressed, a potential race condition in Gutenberg can cause <PostTypeSupportCheck supportKeys="title"> to render titles even though it's not actively supported by the post type. We should only flag the editor as ready once we can successfully resolve the post type.

Closes #3414978

css/components/_block-editor.scss

@@ -47,8 +47,3 @@ ul.block-editor-block-navigation__list {
   padding: 0;
   list-style: none;
 }
-
-// Hide editor's title.
-.wp-block-post-title {
-  display: none;
-}