Remove CreateAccountSigner function and update the unit tests

[urvisavla]

What problem does your feature solve?

After the change in #5105 to use FastBatchInsertBuilder in SignersProcessor, the CreateAccountSigner function is no longer used anywhere except for unit tests. This function should be removed.

What would you like to see?

Update the unit tests that are currently using CreateAccountSigner to use AccountSignersBatchInsertBuilder and then delete the function.

What alternatives are there?

N/A