Delete deprecated method Node::siblings.
Delete deprecated method Node::siblings.

PiperOrigin-RevId: 356862504

monetate/closure-compiler,nawawi/closure-compiler,nawawi/closure-compiler,monetate/closure-compiler,nawawi/closure-compiler,monetate/closure-compiler,ChadKillingsworth/closure-compiler,ChadKillingsworth/closure-compiler,nawawi/closure-compiler,google/closure-compiler,google/closure-compiler,monetate/closure-compiler,ChadKillingsworth/closure-compiler,google/closure-compiler,google/closure-compiler,ChadKillingsworth/closure-compiler