Remove themes.default from i18n-tasks unused check
Remove themes.default from i18n-tasks unused check

This translation key comes in from tootsuite, but the flavours system
does not use it.

glitch-soc/mastodon,vahnj/mastodon,glitch-soc/mastodon,im-in-space/mastodon,Kirishima21/mastodon,Kirishima21/mastodon,Kirishima21/mastodon,glitch-soc/mastodon,vahnj/mastodon,im-in-space/mastodon,vahnj/mastodon,im-in-space/mastodon,Kirishima21/mastodon,glitch-soc/mastodon,vahnj/mastodon,im-in-space/mastodon,vahnj/mastodon