DO NOT MERGE - Add a guard against deleting AVB_PUBKey.

The avb_pubkey may not be present, if the apex was initially
unsigned or generated from a bundle.  In this case, running
sign_apex to generate a signed apex binary would result in an error.
This fix checks for presense of avbpubkey before attempting the
deletion

BUG: 139994107
Change-Id: I3cb2e88a11ad8797e38ba5fb98c96a4ec4135fc8