generate_test_keys: do not quit script if output file already exists

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/scripts/generate_test_keys.py b/tests/scripts/generate_test_keys.py
index 6aac230..67e2abc 100755
--- a/tests/scripts/generate_test_keys.py
+++ b/tests/scripts/generate_test_keys.py
@@ -100,10 +100,6 @@
     args = argparser.parse_args()
 
     output_file = args.output
-    # If the output file already exists, then we can quit (successfully)
-    if os.path.exists(output_file):
-        return
-
     output_file = open(output_file, 'at')
     output_file.write(
         "/*********************************************************************************\n" +