In the end, the giant claw of an oversize backhoe needed only hours to do what 18 years of litigation, hundreds of thousands of dollars in legal fees, and a bottomless well of angst and acrimony had been unable to accomplish before yesterday. Beginning about 7:15 a.m., by court order, Wayne Johnson’s 5,000-square-foot, million-dollar house overlooking Marblehead Harbor finally came tumbling down. Johnson, a financial adviser in his 70s, is moving to a rental apartment in Salem.