ART: Do not use boot image for space gtests.

Using the boot image breaks one test for the SS GC.

This is a partial revert of
    https://android-review.googlesource.com/2253695 .

Test: ART_DEFAULT_GC_TYPE=SS ART_USE_TLAB=true \
      ART_USE_READ_BARRIER=false m test-art-host-gtest
Change-Id: Ib898bc7532ed6052262796d433a872900a3a7f0b
diff --git a/runtime/gc/space/space_test.h b/runtime/gc/space/space_test.h
index f0aead1..4b01e83 100644
--- a/runtime/gc/space/space_test.h
+++ b/runtime/gc/space/space_test.h
@@ -38,10 +38,6 @@
 template <class Super>
 class SpaceTest : public Super {
  public:
-  SpaceTest() {
-    this->use_boot_image_ = true;  // Make the Runtime creation cheaper.
-  }
-
   jobject byte_array_class_ = nullptr;
 
   void AddSpace(ContinuousSpace* space, bool revoke = true) {