Remove second goroutine that calls writeLoop()

I had added the second goroutine for symmetry with `CrawlURL()`[1], but
on second thought there's no need for this. Crawling is time-consuming
whereas writing to disk should be much less so.

[1]:
https://github.com/alphagov/govuk_crawler_worker/blob/a4559142407cbfe50956a777f28f5d96693fa934/workflow.go#L70-L71