Break testnet difficulty reset out of never matching condition.

Testnet resets difficulty if after 2 minutes no block has been
found. However, this feature was implemented with the condition
to only trigger on blocks where no retargeting is done. Since
the introduction of DigiShield, we retarget at every block,
making this condition never to be matched. This commit removes
that condition.

WARNING: THIS COMMIT HARDFORKS THE DOGECOIN TESTNET!

The main network is not affected by this change.
Merge pull request #589 from patricklodder/1.8-dev-testnet-retarget

Break testnet difficulty reset out of never matching condition.