I found this method in the context of a ticket to refactor Paris code to
Doctrine. As far as I can tell, it is not used anymore and it is
safe to delete it.
It was added in 1af5802 and was used only inside
Models\Subscriber::subscribe(). Then subscribe() was moved to
Subscriber\SubscriberActions in 7528f0f. Finally, commit 7db2384
refactored SubscriberActions removing the only two calls to
Source::setSource().
The test class SourceTest is removed as well as its only purpose was to
test Source::setSource().
[MAILPOET-5345]
