Revert "IWYU for tests starting with 'D'."

This reverts commit b5f2897ab5be37e86ccce4ed474a2e02001a9040.

Reason for revert: looks like this broke the build for DM in Google3.  Need more include dirs there?

Original change's description:
> IWYU for tests starting with 'D'.
> 
> Change-Id: I9189e4b56ce1635b627119733447c2ed4220753d
> Reviewed-on: https://skia-review.googlesource.com/129319
> Reviewed-by: Herb Derby <herb@google.com>
> Commit-Queue: Ben Wagner <bungeman@google.com>

TBR=bsalomon@google.com,bungeman@google.com,herb@google.com

Change-Id: I01aac7dae6114685a0652cbde3defccc8a42caea
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/129443
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/tests/DiscardableMemoryTest.cpp b/tests/DiscardableMemoryTest.cpp
index 8447e4d..2b09533 100644
--- a/tests/DiscardableMemoryTest.cpp
+++ b/tests/DiscardableMemoryTest.cpp
@@ -5,13 +5,9 @@
  * found in the LICENSE file.
  */
 
-#include "SkDiscardableMemory.h"
 #include "SkDiscardableMemoryPool.h"
-#include "SkRefCnt.h"
-#include "Test.h"
 
-#include <cstring>
-#include <memory>
+#include "Test.h"
 
 namespace {
 constexpr char kTestString[] = "HELLO, WORLD!";