Revert "Merge pull request #317 from prometheus/fix/miekg-dns-for-srv"

This reverts commit e3bc6fc9dce7aeda995fd4a1bc9d3ccb09b46bff, reversing
changes made to 1cf9e5840a90c358f79cf16d994d120fc14d6615.

Conflicts:
	retrieval/target_provider.go

Change-Id: Icb6e98fb30419e9e2fe9b686c243702ced372014