Strip image-format arg in dex2oat

Avoids printing it out to prevent spam.

Bug: 22858531

Change-Id: I9d0b35b8fa317bc39ab27946baa88812a32381e2
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index b3e3ba6..d30f697 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -126,6 +126,11 @@
       continue;
     }
 
+    // The image format is dropped.
+    if (StartsWith(original_argv[i], "--image-format=")) {
+      continue;
+    }
+
     // This should leave any dex-file and oat-file options, describing what we compiled.
 
     // However, we prefer to drop this when we saw --zip-fd.