Remove subtransactions from Vulnerabilities::HistoricalStatistic

One item was identified as a short term opportunity to alleviate subtransactions' impact to GitLab.com availability in &6540 (comment 648749714)

The most common use of subtransactions appears to be this pattern:

SomeActiveRecordModel.safe_ensure_unique(retries: 1) do
  record = SomeActiveRecordModel.find_or_initialize_by(some_attributes)
  <do stuff to record>

For example:

project = Project.first

ApplicationRecord.transaction do 
  ::Vulnerabilities::HistoricalStatistic.safe_ensure_unique(retries: 1) do
     project.vulnerability_historical_statistics.find_or_initialize_by(date: Time.now) 
  end
end

This effectively translates to:

BEGIN
SAVEPOINT
SELECT * FROM vulnerability_historical_statistics WHERE xxx LIMIT 1
RELEASE SAVEPOINT
COMMIT

/cc @stanhu @tstadelhofer @twoodham