FF Removal: generic_alert_fingerprinting

What does this MR do?

This is to removal the generic_alert_fingerprinting Feature Flag, which is currently flagging the 'GitLab defined alert fingerprinting #214557 (closed)'.

• Feature MR: !34820 (merged)
• Docs MR: !35815 (merged)
• FF enabled in prod (Slack): https://gitlab.slack.com/archives/C101F3796/p1594069859133800

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #224113 (closed)