Trim Heading Block CSS

[bgardner]

Per https://github.com/WordPress/gutenberg/pull/42122, we should be able to consolidate:

.wp-block-post-content h1 + *,
.wp-block-post-content h2 + *,
.wp-block-post-content h3 + *,
.wp-block-post-content h4 + *,
.wp-block-post-content h5 + *,
.wp-block-post-content h6 + * {
    margin-top: 20px;
}

To this:

.wp-block-post-content .wp-block-heading + * {
    margin-top: 20px;
}