Revert "repo-updater: mitigate the possibility of migration deadlock due to rate limit (#3617)"

Created by: tsenart

This reverts commit 1033f67435918f74e8969285117e9c515dbb6f92.

Rationale: https://github.com/sourcegraph/sourcegraph/issues/3624#issuecomment-487000067 Fixes #3624 (closed)