Remove migrate_bio_to_user_details FF

config/feature_flags/development/migrate_bio_to_user_details.yml

----
-name: migrate_bio_to_user_details
-introduced_by_url: 
-rollout_issue_url: 
-group: 
-type: development
-default_enabled: true

db/post_migrate/20200511083541_cleanup_projects_with_missing_namespace.rb

@@ -66,8 +66,6 @@ class CleanupProjectsWithMissingNamespace < ActiveRecord::Migration[6.0]
     end
 
     def ensure_bio_is_assigned_to_user_details
-      return if Feature.disabled?(:migrate_bio_to_user_details, default_enabled: true)
-
       user_detail.bio = bio.to_s[0...255]
     end
 

lib/gitlab/background_migration/migrate_users_bio_to_user_details.rb

@@ -13,8 +13,6 @@ module Gitlab
       end
 
       def perform(start_id, stop_id)
-        return if Feature.disabled?(:migrate_bio_to_user_details, default_enabled: true)
-
         relation = User
           .select("id AS user_id", "substring(COALESCE(bio, '') from 1 for 255) AS bio")
           .where("(COALESCE(bio, '') IS DISTINCT FROM '')")

spec/lib/gitlab/background_migration/migrate_users_bio_to_user_details_spec.rb

@@ -82,21 +82,4 @@ RSpec.describe Gitlab::BackgroundMigration::MigrateUsersBioToUserDetails, :migra
 
     expect(user_detail).to be_nil
   end
-
-  context 'when `migrate_bio_to_user_details` feature flag is off' do
-    before do
-      stub_feature_flags(migrate_bio_to_user_details: false)
-    end
-
-    it 'does nothing' do
-      already_existing_user_details = user_details.where(user_id: [
-        user_has_different_details.id,
-        user_already_has_details.id
-      ])
-
-      subject
-
-      expect(user_details.all).to match_array(already_existing_user_details)
-    end
-  end
 end