DEV: Remove JoyPixels emoji option (#12197)

- removes the option from site settings
- deletes the site setting on existing sites that have it
- marks posts using emojis as requiring a rebake

Note that the actual image files are not removed here, the plan is to
remove them in a few weeks/months (when presumably the rebaking of old
posts has been completed).

app/models/emoji_set_site_setting.rb

@@ -13,7 +13,6 @@ class EmojiSetSiteSetting < EnumSiteSetting
       { name: 'emoji_set.apple_international', value: 'apple' },
       { name: 'emoji_set.google', value: 'google' },
       { name: 'emoji_set.twitter', value: 'twitter' },
-      { name: 'emoji_set.emoji_one', value: 'emoji_one' },
       { name: 'emoji_set.win10', value: 'win10' },
       { name: 'emoji_set.google_classic', value: 'google_classic' },
       { name: 'emoji_set.facebook_messenger', value: 'facebook_messenger' },