stop adding ota-rel-keys tag when signing

ota-rel-keys is now redundant with release-keys, since there are no
"half-signed" builds any more.
diff --git a/tools/releasetools/sign_target_files_apks b/tools/releasetools/sign_target_files_apks
index 961e643..9d296d8 100755
--- a/tools/releasetools/sign_target_files_apks
+++ b/tools/releasetools/sign_target_files_apks
@@ -55,7 +55,7 @@
       the last component of the build fingerprint).  Prefix each with
       '+' or '-' to indicate whether that tag should be added or
       removed.  Changes are processed in the order they appear.
-      Default value is "-test-keys,+ota-rel-keys,+release-keys".
+      Default value is "-test-keys,+release-keys".
 
 """
 
@@ -80,7 +80,7 @@
 OPTIONS.extra_apks = {}
 OPTIONS.key_map = {}
 OPTIONS.replace_ota_keys = False
-OPTIONS.tag_changes = ("-test-keys", "+ota-rel-keys", "+release-keys")
+OPTIONS.tag_changes = ("-test-keys", "+release-keys")
 
 def GetApkCerts(tf_zip):
   certmap = {}