Remove unsupported var that causes error PLAT-1769

The COMPREHENSIVE_THEME_DIR variable is unsupported as of
https://github.com/edx/edx-platform/pull/16289

If this variable is not an empty string edxapp will refuse to run
and print an error message.

playbooks/roles/edxapp/defaults/main.yml

@@ -1183,9 +1183,6 @@ generic_env_config:  &edxapp_generic_env
   VIDEO_TRANSCRIPTS_SETTINGS: "{{ EDXAPP_VIDEO_TRANSCRIPTS_SETTINGS }}"
   BLOCK_STRUCTURES_SETTINGS: "{{ EDXAPP_BLOCK_STRUCTURES_SETTINGS }}"
 
-  # Deprecated, maintained for backward compatibility
-  COMPREHENSIVE_THEME_DIR: "{{ EDXAPP_COMPREHENSIVE_THEME_DIR }}"
-
   COMPREHENSIVE_THEME_DIRS: "{{ EDXAPP_COMPREHENSIVE_THEME_DIRS }}"
   COMPREHENSIVE_THEME_LOCALE_PATHS: "{{ EDXAPP_COMPREHENSIVE_THEME_LOCALE_PATHS }}"
   ENABLE_COMPREHENSIVE_THEMING: "{{ EDXAPP_ENABLE_COMPREHENSIVE_THEMING }}"