test-api: Remove spurious % formatting.

BUG=none
TEST=unittest

Change-Id: Id2a4f281fa3841640f45b8940482f78d535251bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3289747
Tested-by: Sean McAllister <smcallis@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/api/controller/test.py b/api/controller/test.py
index 830eae0..b4a3b26 100644
--- a/api/controller/test.py
+++ b/api/controller/test.py
@@ -230,7 +230,7 @@
   # Repeated . and - aren't allowed
   for char in '.-':
     if char * 2 in key:
-      return f"'{char}' can\'t be repeated in label key" % char
+      return f"'{char}' can\'t be repeated in label key"
 
 
 @faux.success(_BuildTestServiceContainersResponse)