Revert "Update include path to date.h which was moved in the meanwhile"

This reverts commit 3e118baf.

Reason for revert: Reverting as ftang@'s CL here will be reverted. 
https://chromium-review.googlesource.com/c/v8/v8/+/1601370, and this was a fix for a bad landing.

Original change's description:
> Update include path to date.h which was moved in the meanwhile
> 
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: chromium:950851
> Change-Id: Ia5e547e7cd7947f91814cb24de3dcf3b822905da
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1611811
> Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
> Commit-Queue: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#61552}

TBR=gdeepti@chromium.org,ishell@chromium.org

Change-Id: Ibe1671b1973afae8db6180d3f5471763786bd23d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:950851
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1613637Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61553}

test/unittests/date-cache-unittest.cc

 // Copyright 2019 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-
 #include "src/base/platform/platform.h"
-
-#include "src/date/date.h"
+#include "src/date.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "unicode/strenum.h"
 #include "unicode/timezone.h"