fix: set default theme by simply deleting SiteTheme objects

`tutor ... do settheme default` is meant to revert to the
default theme.  However, in its current implementation, it
creates SiteTheme objects pointing to a theme named "default",
which doesn't exist, resulting in errors like:

    Theme dirs:
    [Path('/openedx/themes')]]
    Traceback (most recent call last):
      File "/openedx/edx-platform/openedx/core/djangoapps/theming/helpers.py", line 204, in get_current_theme
	themes_base_dir=get_theme_base_dir(site_theme.theme_dir_name),
      File "/openedx/edx-platform/openedx/core/djangoapps/theming/helpers.py", line 242, in get_theme_base_dir
	raise ValueError(
    ValueError: Theme 'default' not found in any of the following themes dirs,

This works from the perspective of the user, because a missing theme is
treated as the default theme. However, the errors are unneccesary &
confusing.

By simply deleting & not recreating SiteTheme objects instead,
we are able to revert to the default theme while keeping the
logs clear of theming errors.

changelog.d/20230714_100627_kyle.md

@@ -0,0 +1 @@
+- [Bugfix] Improve `tutor ... do settheme default` so that it reverts to the default theme rather than trying to switch to a nonexistent theme named "default". This will clear up some error noise from LMS/CMS logs. (by @kdmccormick)

tutor/commands/jobs.py

@@ -216,7 +216,8 @@ def assign_theme(name, domain):
         site.name = domain[:name_max_length]
         site.save()
     site.themes.all().delete()
-    site.themes.create(theme_dir_name=name)
+    if name != 'default':
+        site.themes.create(theme_dir_name=name)
 """
     domain_names = domain_names or [
         "{{ LMS_HOST }}",