Delete defunct ExtensionSchemaFull paths.

These paths are 100% unreachable today via our public APIs (they're only reachable by our tests which pierce-the-veil of the package-protected api). The only entry point is via GeneratedMessageLite, and this code already checks for GeneratedMessageLite types and always uses the Lite types.

This behavior means that even if someone in OSS uses Lite gencode on Full runtime this path is never reachable today.

PiperOrigin-RevId: 917409414