Actually the said letter from the bank should be enough for the registrar to record removal of encumbrance on the property. When the authority is insisting for personal attendance of the bank representative, you give a written notice to the bank stating the necessity. Its duty of the bank to see all the formalities of the loan are over after repayment. Bank failing to attend your call comes under deficiency of service and you can sue the bank in consumer forum.
Or takeup the matter (representation on paper) to the notice of next higher authority in the Registration dept. Even there if you are advised to bring the bank representative in person, request for a written order.
This is such a routine procedure, the bank and the authority must have clarity. Public should not be troubled for their ignorance of procedures. Make them wakeup.