Remove obsolete code

This removes code that was only applicable to older API versions (<15) which are no longer supported.

There are a few preferences in `AccountSettings` and `GlobalSettings` that were either deleted or their default values changed.  I see that the code permits multiple versions for a setting with different default values or `null` indicating the setting was removed, but I only find a couple instances where this capability was actually used, e.g.:

        s.put("fontSizeMessageViewContent", Settings.versions(
                new V(1, new WebFontSizeSetting(3)),
                new V(31, null)
            ));

It seems that most of the time, the practice has been to directly delete the preference or change the default without creating a second version.  That is what I have done here.

Also, I have not incremented `Settings.VERSION`.  From what I see, doing that would only prevent users from exporting their settings from this newest version (wherein a few settings are now deleted) and importing the settings into a prior version (wherein the deleted settings would be created anew because they were not found in the export file).  I see no harm in letting users do that if they choose.

Let me know if the choices I've made are not appropriate.