Revert "feat: custom theme loading"

This reverts commit 3b9d0237b2.

This doesn't work because we can't get into the root account in the
entrypoint and we need that to use microdnf. Another approach is needed.

.env.sample

@@ -7,9 +7,6 @@ LETS_ENCRYPT_ENV=production
 
 ADMIN_USERNAME=admin
 
-# CUSTOM_THEME_ENABLED=1
-# CUSTOM_THEME_URL=
-
 SECRET_DB_ROOT_PASSWORD_VERSION=v1
 SECRET_DB_PASSWORD_VERSION=v1
 SECRET_ADMIN_PASSWORD_VERSION=v1