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