Remove Bidding Signals Format "V1" support

This removes mention of the Trusted Bidding Signals V1 format, as well as the "Bidding-Signals-Format-Version: 2" header, as there's no plan for a version 3, and "version 2" can be easily confused with the KVv2 API.  All responses will be assumed to be version 2.

FLEDGE.md

@@ -765,8 +765,6 @@ The response from the server should be a JSON object of the form:
 }
 ```
 
-and the server must include the HTTP response header `X-fledge-bidding-signals-format-version: 2`.  If the server does not include the header, the response will assumed to be an in older format, where the response is only the contents of the `keys` dictionary.
-
 The value of each key that an interest group has in its `trustedBiddingSignalsKeys` list will be passed from the `keys` dictionary to the interest group's generateBid() function as the `trustedBiddingSignals` parameter. Values missing from the JSON object will be set to null. If the JSON download fails, or there are no `trustedBiddingSignalsKeys` or `trustedBiddingSignalsURL` in the interest group, then the `trustedBiddingSignals` argument to generateBid() will be null.
 
 The `perInterestGroupData` dictionary contains optional data for interest groups whose names were included in the request URL. The `priorityVector` will be used to calculate the final priority for an interest group, if that interest group has `enableBiddingSignalsPrioritization` set to true in its definition. Otherwise, it's only used to filter out interest groups, if the dot product with `prioritySignals` is negative. See [Filtering and Prioritizing Interest Groups](#35-filtering-and-prioritizing-interest-groups) for more information.