_deleted_at incorrectly set to the date the server last started, not the actual deletion time

[jnm]

very common python mistake:

kobocat/onadata/apps/logger/models/instance.py

Lines 377 to 382 in 1973fb0

	def set_deleted(self, deleted_at=timezone.now()):
	self.deleted_at = deleted_at
	self.save()
	# force submission count re-calculation
	self.xform.submission_count(force_update=True)
	self.parsed_instance.save()

[jnm]

deleted_at has been fully removed as of #732 and #744.