Blockbase: remove unused post meta styling

[RavanH]

Proposal

There are several style rules and files that are aimed at a block with class .post-meta. There is no such block used in Blockbase. However, these styles are used in child themes Mayland Blocks and Skatepark. I would argue that these style belong there, not in the parent theme.

Files involved are:
sass/post/_meta.scss
sass/ponyfill.scss (last import)
assets/svg/post-author.svg
assets/svg/post-category.svg
assets/svg/post-date.svg
assets/svg/post-tag.svg (already missing!)

Of course, the child themes in question would need to set up their own version of:

	.wp-block-post-author,
	.wp-block-post-date,
	.wp-block-post-tags,
	.wp-block-post-hierarchical-terms {
		display: flex;
		margin-right: calc(2 * var(--wp--custom--margin--baseline) );
	}

	.wp-block-post-author:before,
	.wp-block-post-date:before,
	.wp-block-post-tags:before,
	.wp-block-post-hierarchical-terms:before {
		align-self: center;
		content: '';
		display: inline-block;
		margin-right: calc(0.5 * var(--wp--custom--margin--baseline) );
		height: 16px;
		width: 16px;
		background-color: currentColor;
	}

Alternative: fix missing post-tag.svg, fix .wp-block-post-tags (should be .wp-block-post-terms) and move the scss out of the post-meta class so it will work independently for the Post Author/Date/Categories/Tags blocks that can be added in posts or pages.

[scruffian]

I think I'd prefer to add post meta to blockbase :)

[MaggieCabrera]

Thanks for this PR, I totally agree that if the css is not used by the theme it shouldn't be there, but since all the children use it, I think it makes a compelling argument that Blockbase should include it too, since it's a pattern that's widely used all over. @kjellr do you think we should add this to Blockbase? This is how it looks like in Seedlet blocks:

[kjellr]

Yeah, I think it makes sense to add it to blockbase — even if it just houses the post date to start.