X509CertImpl: comment out unused code

Otherwise, whenever this code is changed upstream we'll go
through the process of checking how the change affects
behaviour (just to find that the code is unused).

There's no point to have that maintenance burden for unused
code.

Test: make droid
Bug: 29631070
Change-Id: I6c886e2939b669d960a5f4889ec9f4c4eb9b4682