Audit and potentially remove custom image sizes

[justintadlock]

The theme has long needed to register custom image sizes using the standard add_image_size() function. The reason for this was essentially do the job of custom aspect ratios, which are landing in WordPress 6.6.

Unfortunately, the Image block (unlike the Post Featured Image and Cover blocks) does not support aspect ratios when set to wide or full width. There is an open ticket for this, but it won't land in 6.6:

• Add support for aspect ratio on wide and full images WordPress/gutenberg#53652

The theme itself likely only needs aspect ratios for Post Featured Image and Cover, so we may be able to do this before shipping alongside 6.6. But it would limit user choice until the above ticket can land in WP.

Additionally, this may be a good time to audit the theme's image sizes now that we have true aspect ratios.

[acwi111]

[justintadlock]

This was addressed in ad245fa and e6958ef.