MDL-51580 lang strings: Delete unused lang strings
With previous commit some lang strings became unused. As far as they
are not reusable we are deleting them plain and straight. This is the
list of strings, for reference:
- orphanedquestionscategory
- orphanedquestionscategoryinfo
- oauth2upgrade_message_small
- oauth2upgrade_message_subject
- oauth2upgrade_message_content
- apiv1migration_message_small
- apiv1migration_message_subject
- apiv1migration_message_content
- security_key_notice_message_small
- security_key_notice_message_subject
- security_key_notice_message_content
- pendingupgrades_message_small
- pendingupgrades_message_subject
- pendingupgrades_message_content
