Delete observation API endpoint

[DELETE] /api/balances/{address}/observation

Should forget the wallet address and stop observing its balance.

Errors:
■ 204 No content - specified address is not observed

Python implementation

```py
@api.route('/balances//observation', methods=['DELETE'])
def delete_observation(address):
"""
Delete the specified address from observation list
"""

result = delete_address_observation(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 ""

```