vboot: no need to compile C++ files in Makefile
vboot: no need to compile C++ files in Makefile

The last remaining C++ files were removed in CL:367882.
vboot_reference does not have any C++ files anymore, and
thus does not need this rule in its Makefile.

BUG=b:124141368
TEST=make clean && make runtests
BRANCH=none

Change-Id: Id59b2e593b1748293f045e5f7152ec2647ba7342
Signed-off-by: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1780082
Commit-Queue: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@chromium.org>
Tested-by: Joel Kitching <c2feea8c1334a0d339e816ae4c7c9ecfa9b30a91@chromium.org>
Reviewed-by: Julius Werner <4d12f06cff3725783b352157673a720bc67c4afe@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot