Remove v1 IS API fallbacks once servers are updated

[jryans]

While adding support for the IS v2 API, we have added fallbacks that also try the v1 API if v2 is missing. We should remove these once ISes are updated to support v2 APIs.

Search for this issue in the code to find related areas. Fallbacks added as part of:

• Add support for IS v2 API with authentication matrix-org/matrix-js-sdk#1002
• Add support for IS v2 API with authentication matrix-org/matrix-react-sdk#3256