Remove the thread_count=1 hack on target device.

Earlier we set thread_count to 1 on the target device, to
avoid "fork" of dex2oat and reduce physical memory usage.

However, since we have multithreading now, we can remove
this code, and take adventage of multi-cores.

Change-Id: I767cb98630d6995ca3eb7fcba938c9710346ab6c
diff --git a/src/dex2oat.cc b/src/dex2oat.cc
index 42a3266..0d3fa58 100644
--- a/src/dex2oat.cc
+++ b/src/dex2oat.cc
@@ -561,12 +561,6 @@
   thread_count = 1;
 #endif
 
-#if defined(ART_USE_LLVM_COMPILER) && defined(ART_TARGET)
-  // To avoid high memory usage, always run dex2oat in single thread mode when
-  // we are using LLVM-based compiler.
-  thread_count = 1;
-#endif
-
   if (oat_filename.empty() && oat_fd == -1) {
     Usage("Output must be supplied with either --oat-file or --oat-fd");
   }