[ci] remove reference to non-existent phantom file (#29541)

Phantom support was removed recently, need to remove this reference as well
otherwise the `packer_cache` script fails when building new CI images.

Related: #27141, #27142

.ci/packer_cache.sh

@@ -21,6 +21,5 @@ tar -cf "$HOME/.kibana/bootstrap_cache/master.tar" \
   x-pack/node_modules \
   x-pack/plugins/*/node_modules \
   x-pack/plugins/reporting/.chromium \
-  x-pack/plugins/reporting/.phantom \
   test/plugin_functional/plugins/*/node_modules \
   .es;