In your kernel configuration file you will find this line:

CONFIG_SYSTEM_TRUSTED_KEYS="debian/canonical-certs.pem"

Change it to this:

CONFIG_SYSTEM_TRUSTED_KEYS=""

Depending on your source structure you might be able to do it via command line. Examples:

scripts/config --disable SYSTEM_TRUSTED_KEYS

or

scripts/config --set-str SYSTEM_TRUSTED_KEYS ""

EDIT: Another key has been added to the default Canonical kernel configuration since this answer was posted:

CONFIG_SYSTEM_REVOCATION_KEYS="debian/canonical-revoked-certs.pem"

So, it also needs to be dealt with for user kernel compiles to complete:

scripts/config --disable SYSTEM_REVOCATION_KEYS

See also git based mainline kernel compile notes.