OvmfPkg/IgvmSecureBootDxe: Remove CustomMode code

Just require PcdRequireSelfSignedPk=FALSE instead of using CustomMode to
enroll PK keys which are not self-signed.

PcdRequireSelfSignedPk=FALSE is the default configuration for OVMF since
commit 3c01a11daae2 ("OvmfPkg: set PcdRequireSelfSignedPk to FALSE").

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>