Remove unresolved classes from image classes.

This allows compiling partial boot image, without conscrypt,
while using the current image classes in preloaded-classes.
Excluding the conscrypt makes the class
  android.security.net.config.TrustedCertificateStoreAdapter
unresolved because it extends the class
  com.android.org.conscrypt.TrustedCertificateStore .
Without pruning, we hit a DCHECK() in ImageWriter, checking
that image classes are not erroneous. And we clearly do not
want to put erroneous classes in the partial boot image.

Test: Build partial boot image without conscrypt.
Bug: 119868597
Change-Id: I6017462366a4b9e69abe1fb3d22461cab35f84c5