xds/googlec2p: cleanup in googlec2p resolver.

[Pranjali-2501]

The xdsclient package contains legacy support for a "fallback" bootstrap configuration, which was originally used by the googlec2p resolver via SetFallbackBootstrapConfig.
This global fallback mechanism has been superseded by the changes in PR #8648. The googlec2p resolver now uses this new, explicit approach to create a xdsClient in googlec2p resolver.

To simplify the API and prevent confusion, the legacy fallback logic should be removed.

• Deprecate and remove the exported xdsclient.SetFallbackBootstrapConfig function.
• Remove the internal fallbackConfig field from the xdsclient.Pool and all associated logic that applies it during client creation.