Remove uvm extension from unimplemented features

Author: emlun

webauthn-server-core/README

@@ -16,8 +16,6 @@ it.
 * Attestation statement formats:
  ** https://www.w3.org/TR/webauthn/#sctn-tpm-attestation[`tpm`]
  ** https://www.w3.org/TR/webauthn/#sctn-android-key-attestation[`android-key`]
-* Extensions:
- ** https://www.w3.org/TR/webauthn/#sctn-uvm-extension[`uvm`]
 
 
 == Won't-implement features