Revert "ART: Unlink target oat file before compiling"

Change to the old code before changing CreateEmptyFile.

This reverts commit 4591ae27de90988fbcb9af0c942e633f97ab454e.

Bug: 22047255
diff --git a/dex2oat/ b/dex2oat/
index 74d5c0c..7a23746 100644
--- a/dex2oat/
+++ b/dex2oat/
@@ -1038,10 +1038,6 @@
   bool OpenFile() {
     bool create_file = !oat_unstripped_.empty();  // as opposed to using open file descriptor
     if (create_file) {
-      // We're supposed to create this file. If the file already exists, it may be in use currently.
-      // We must not change the content of that file, then. So unlink it first.
-      unlink(oat_unstripped_.c_str());
       if (oat_location_.empty()) {
         oat_location_ = oat_filename_;