Remove the returnGroup argument and logic from signTransactions

[drichar]

This is a holdover from the original MyAlgo implementation (pre ARC-0001) that only supported signatures by a single account per group.

NFDomains requires signatures from two accounts to verify a linked address, so our solution was to return the entire group (not just the signed transactions) so a second signing request could be initiated.

This is no longer necessary, and adds unnecessary complexity to the library's signing logic. It also breaks with the ARC-0001 SignTxnsFunction type signature by returning unsigned transactions.