buildkitd: Do not enable macspoofchk CNI setting

NOTE this option was not in the buildkitd referenced configuration. I added it after reading the CNI reference but didn't realize it required an additional nft binary to be present. Use of it results in the following buildkitd startup error upon deployment:

failed to setup spoof-check: failed to execute nft nft -j -f /tmp/spoofcheck-3647386428: exec: "nft": executable file not found in $PATH

Bug: T329213