Allow delete permissions in Gerrit ACLs

We now have the option to assign delete permissions to groups in
Gerrit, which would grant them the ability to delete branches
through the WebUI or API. Since this is a new setting, it was not
previously recognized by our linter. Extend it so that we won't
raise an error if this appears in an ACL.

Change-Id: I2b182d31e3ca5809a53aec851015341f2e67825d

tools/normalize_acl.py

@@ -64,6 +64,7 @@ valid_keys = {'abandon',
               'copyMinScore',
               'create',
               'defaultValue',
+              'delete',
               'editHashtags',
               'exclusiveGroupPermissions',
               'forgeAuthor',