Remove default theme

[ufundo]

Overview

Remove the theme level default theme handling.

Follow up on switching default to Minetta at setting level.

Before

• there's a default for theme settings at the setting level "default"
• there's a default for theme at the theme level, which gives you Greenwich
• theme options list in Display Settings includes an extra option "Automatic" which gives you Greenwich
• if you select a theme from an extension, then disable the extension, you'll get Greenwich
• there's a hard dependency on Greenwich being an option, and Greenwich extension is required (and hidden)

After

• theme options list in Display Settings only includes explicit choices for the themes that are available
• you can enable/disable whatever extensions you want (including Greenwich)
• if you select a theme from an extension, then disable the extension, you'll get unthemed - and (realise what you've done and do something about it)