Remove unused return values in Lists

[Alberto Cuesta Cañada]

For all of these, remove the returns(...) in the function declaration.

RankedList.insert(uint256,address) (drafts/lists/RankedList.sol#73-84) has missing return statements
RankedList._addHead(uint256,address) (drafts/lists/RankedList.sol#115-123) has missing return statements
RankedList._addTail(uint256,address) (drafts/lists/RankedList.sol#128-140) has missing return statements
RankedList._insertAfter(uint256,uint256,address) (drafts/lists/RankedList.sol#145-159) has missing return statements
RankedList._insertBefore(uint256,uint256,address) (drafts/lists/RankedList.sol#164-174) has missing return statements
LinkedList.addHead(address) (lists/LinkedList.sol#95-102) has missing return statements
LinkedList.addTail(address) (lists/LinkedList.sol#107-119) has missing return statements
LinkedList.insertAfter(uint256,address) (lists/LinkedList.sol#142-150) has missing return statements
LinkedList.insertBefore(uint256,address) (lists/LinkedList.sol#155-166) has missing return statements
DoubleLinkedList.addHead(address) (lists/DoubleLinkedList.sol#68-76) has missing return statements
DoubleLinkedList.addTail(address) (lists/DoubleLinkedList.sol#81-93) has missing return statements
DoubleLinkedList.insertAfter(uint256,address) (lists/DoubleLinkedList.sol#124-138) has missing return statements
DoubleLinkedList.insertBefore(uint256,address) (lists/DoubleLinkedList.sol#143-153) has missing return statements