nix(loadtest): remove noise from report

Ultimately, we only look at the `rate` column, so we can just as well
remove all other columns.

This makes the next step, when we split results by request type, much
less noisy.

nix/tools/loadtest.nix

@@ -282,8 +282,7 @@ let
 
         pd.read_json(sys.stdin) \
           .set_index('param') \
-          .drop(['branch', 'earliest', 'end', 'latest']) \
-          .fillna("") \
+          .loc['rate'] \
           .convert_dtypes() \
           .to_markdown(sys.stdout, floatfmt='.0f')
       '';