Remove redundant RenderPass in Dawn unittests

This patch removes all the redundant creations of RenderPass in Dawn
unittests. In these tests render passes are actually never used.

BUG=dawn:6

Change-Id: I6301a150ea21af1a7ccc3b7ce2dd78351eb7e179
Reviewed-on: https://dawn-review.googlesource.com/c/4720
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>

src/tests/unittests/validation/RenderPipelineValidationTests.cpp

@@ -23,8 +23,6 @@ class RenderPipelineValidationTest : public ValidationTest {
         void SetUp() override {
             ValidationTest::SetUp();
 
-            renderpass = CreateSimpleRenderPass();
-
             vsModule = utils::CreateShaderModule(device, dawn::ShaderStage::Vertex, R"(
                 #version 450
                 void main() {
@@ -40,7 +38,6 @@ class RenderPipelineValidationTest : public ValidationTest {
                 })");
         }
 
-        dawn::RenderPassDescriptor renderpass;
         dawn::ShaderModule vsModule;
         dawn::ShaderModule fsModule;
 };