[api] Stop observation to address

[DELETE] /api/transactions/history/to/{address}/observation

Should stop observation of the transactions that transfer fund to the address . Should affect result of the `[GET] /api/transactions/history/to/{address}` .

Errors:

* `204 No content` : transactions to the address are not observed.

Python implementation

```py

@api.route('/transactions/history/to//observation', methods=['DELETE'])
def del_history_to_address(address):
"""
Stops observation of the transactions that transfer fund to the address
"""

result = delete_transaction_observation_to_address(address)

# if successfully deleted from observation list, return a plain 200
if "error" in result:
return make_response(jsonify(build_error(result["error"])), result["status"])
else:
return ""

```