Don't force the boot image to using quick.

Instead, put the debuggable flag.

Also, remove obsolete ART_USE_OPTIMIZING_COMPILER environment
variable.

Change-Id: Idde00da0d063aa461faa0308134f3b420de9fe0a
diff --git a/compiler/common_compiler_test.h b/compiler/common_compiler_test.h
index dc2bc5c..67b4428 100644
--- a/compiler/common_compiler_test.h
+++ b/compiler/common_compiler_test.h
@@ -92,7 +92,7 @@
 
   void UnreserveImageSpace();
 
-  Compiler::Kind compiler_kind_ = kUseOptimizingCompiler ? Compiler::kOptimizing : Compiler::kQuick;
+  Compiler::Kind compiler_kind_ = Compiler::kOptimizing;
   std::unique_ptr<CompilerOptions> compiler_options_;
   std::unique_ptr<VerificationResults> verification_results_;
   std::unique_ptr<DexFileToMethodInlinerMap> method_inliner_map_;