Fix backups not working when feature_flags table does not exist

What does this MR do and why?

Previously this would fail if you attempted to restore the database:

RAILS_ENV=test bundle exec rake db:reset
RAILS_ENV=test bundle exec rake gitlab:backup:db:create
RAILS_ENV=test bundle exec rake gitlab:db:drop_tables
RAILS_ENV=test bundle exec rake gitlab:backup:db:restore

The last step would attempt to read backup_information.yml and fail because the file isn't generated in the gitlab:backup:db:create task. We can disable the reading of that file since it's only needed in the gitlab:backup:create task.

In addition, the third step is optional. Since the feature_flags table does not exist, the feature flag check that happened in the last step would fail.

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