Gutenberg: Remove redundant templates/parts

[iandunn]

There are cases where we've had to create new templates and template parts that are almost identical to existing ones, because we can't have simple logic inside the HTML files. Examples:

• index.html and category.html are identical except for a few class slugs
• content-post.html and content-page.html are identical except for the post terms. They could be a single singular.html with an if() around the terms block.

Once WordPress/gutenberg#32939 is resolved, we should be able to remove a lot of the WET stuff and make things simpler.

Similarly, there's some of duplicated markup:

• query pagination. we should be able to revert Move pagination from template-part to the main templates #179 once Template parts need to inherit the query context gutenberg#28011 is fixed