Update `strip_optimizer()` (#5949)

Replace 'training_result' with 'best_fitness' in strip_optimizer() to match key with ckpt from train.py

utils/general.py

@@ -738,7 +738,7 @@ def strip_optimizer(f='best.pt', s=''):  # from utils.general import *; strip_op
     x = torch.load(f, map_location=torch.device('cpu'))
     if x.get('ema'):
         x['model'] = x['ema']  # replace model with ema
-    for k in 'optimizer', 'training_results', 'wandb_id', 'ema', 'updates':  # keys
+    for k in 'optimizer', 'best_fitness', 'wandb_id', 'ema', 'updates':  # keys
         x[k] = None
     x['epoch'] = -1
     x['model'].half()  # to FP16