Remove public key pubkey_id from GRUB’s keyring of trusted keys.
pubkey_id is the last four bytes (eight hexadecimal digits) of
the GPG v4 key id, which is also the output of
(see list_trusted). Outside of GRUB, the key id can be obtained
These keys are used to validate signatures when environment variable
check_signatures is set to
(see check_signatures), and by some invocations of
verify_detached (see verify_detached). See Using digital signatures, for more information.