Revert "Kefka: Fix link test error for VbExEcEnteringMode"

This reverts commit 0df5fd4a39d040780003ecbf64c9dd1698d2b011.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Kefka: Fix link test error for VbExEcEnteringMode
> 
> Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
> 
> BUG=b/128455667
> BRANCH=strago
> TEST=emerge-kefka vboot_reference
> 
> Change-Id: Iab3ed8b7f9bff39801b488827e73d972a6237b35
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1669738
> Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
> Commit-Queue: Daisuke Nojiri <dnojiri@chromium.org>
> Tested-by: Daisuke Nojiri <dnojiri@chromium.org>
> Auto-Submit: Daisuke Nojiri <dnojiri@chromium.org>

Bug: b/128455667
Change-Id: Ib8374e57c19a2d12355b7af570a3e2acff445d51
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1720950
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Tested-by: Daisuke Nojiri <dnojiri@chromium.org>
Auto-Submit: Daisuke Nojiri <dnojiri@chromium.org>
Commit-Queue: Daisuke Nojiri <dnojiri@chromium.org>
diff --git a/firmware/stub/vboot_api_stub.c b/firmware/stub/vboot_api_stub.c
index d1b00fa..7320b6c 100644
--- a/firmware/stub/vboot_api_stub.c
+++ b/firmware/stub/vboot_api_stub.c
@@ -173,8 +173,3 @@
 {
 	return 0;
 }
-
-VbError_t VbExEcVbootDone(void)
-{
-	return VBERROR_SUCCESS;
-}