Remove encodeURI call as jquery params method is enough

A couple of zendesk tickets related to this https://govuk.zendesk.com/agent/#/tickets/302399 and https://govuk.zendesk.com/agent/#/tickets/278565
The problem affects date selections in Smartanswers where the ajax error callback is triggered. This was leading to the resulting params being double encoded because of encodeURI + $.param(...).