Remove unused PEARSON_TEST_PASSWORD setting

I'm not seeing this referenced anywhere in the `edx-platform` codebase.

playbooks/roles/edxapp/defaults/main.yml

@@ -322,7 +322,6 @@ EDXAPP_EXTRA_REQUIREMENTS: []
 
 EDXAPP_GOOGLE_ANALYTICS_ACCOUNT: "None"
 
-EDXAPP_PEARSON_TEST_PASSWORD: ""
 EDXAPP_SEGMENT_IO_LMS: false
 EDXAPP_SEGMENT_IO_LMS_KEY: ""
 EDXAPP_OPTIMIZELY_PROJECT_ID: "None"
@@ -786,7 +785,6 @@ generic_env_config:  &edxapp_generic_env
 
 lms_auth_config:
   <<: *edxapp_generic_auth
-  PEARSON_TEST_PASSWORD: "{{ EDXAPP_PEARSON_TEST_PASSWORD }}"
   SEGMENT_IO_LMS_KEY: "{{ EDXAPP_SEGMENT_IO_LMS_KEY }}"
   OPTIMIZELY_PROJECT_ID: "{{ EDXAPP_OPTIMIZELY_PROJECT_ID }}"
   EDX_API_KEY: "{{ EDXAPP_EDX_API_KEY }}"