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/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index cc32da1..680e2d7 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -235,11 +235,7 @@
   UsageError("  --compiler-backend=(Quick|Optimizing): select compiler backend");
   UsageError("      set.");
   UsageError("      Example: --compiler-backend=Optimizing");
-  if (kUseOptimizingCompiler) {
-    UsageError("      Default: Optimizing");
-  } else {
-    UsageError("      Default: Quick");
-  }
+  UsageError("      Default: Optimizing");
   UsageError("");
   UsageError("  --compiler-filter="
                 "(verify-none"
@@ -503,7 +499,7 @@
 class Dex2Oat FINAL {
  public:
   explicit Dex2Oat(TimingLogger* timings) :
-      compiler_kind_(kUseOptimizingCompiler ? Compiler::kOptimizing : Compiler::kQuick),
+      compiler_kind_(Compiler::kOptimizing),
       instruction_set_(kRuntimeISA),
       // Take the default set of instruction features from the build.
       verification_results_(nullptr),
@@ -752,10 +748,9 @@
 
   void ProcessOptions(ParserOptions* parser_options) {
     image_ = (!image_filename_.empty());
-    if (!parser_options->requested_specific_compiler && !kUseOptimizingCompiler) {
-      // If no specific compiler is requested, the current behavior is
-      // to compile the boot image with Quick, and the rest with Optimizing.
-      compiler_kind_ = image_ ? Compiler::kQuick : Compiler::kOptimizing;
+    if (image_) {
+      // We need the boot image to always be debuggable.
+      parser_options->debuggable = true;
     }
 
     if (oat_filename_.empty() && oat_fd_ == -1) {