accounts: remove UpdateAccount and last usages

We remove the last few calls to UpdateAccount from the
TestAccountService test. Previously, the UpdateAccount call was used in
this test to also _insert_ new accounts and to force the account ID to
be set to specific values. So here we need to instead call the store's
NewAccount method to insert a new account, then we get the AccountID
from the returned value and we need to then use this returned ID (which
is no longer forceable) in the remainder of the tests.

Author: ellemouton

accounts/interface.go

@@ -207,11 +207,6 @@ type Store interface {
 		expirationDate time.Time, label string) (
 		*OffChainBalanceAccount, error)
 
-	// UpdateAccount writes an account to the database, overwriting the
-	// existing one if it exists.
-	UpdateAccount(ctx context.Context,
-		account *OffChainBalanceAccount) error
-
 	// Account retrieves an account from the Store and un-marshals it. If
 	// the account cannot be found, then ErrAccNotFound is returned.
 	Account(ctx context.Context, id AccountID) (*OffChainBalanceAccount,