cache: remove unused CompareWithParent interface
cache: remove unused CompareWithParent interface

CompareWithParent is not used in the current code.

Signed-off-by: Akihiro Suda <10ddef0dea2a66ab193b07198faa879adcb213d8@hco.ntt.co.jp>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc