Simplify check_requirements() usage (#4855)

* Simplify `check_requirements()` usage

* remove assert, print()

val.py

@@ -327,7 +327,7 @@ def parse_opt():
 
 def main(opt):
     set_logging()
-    check_requirements(requirements=ROOT / 'requirements.txt', exclude=('tensorboard', 'thop'))
+    check_requirements(exclude=('tensorboard', 'thop'))
 
     if opt.task in ('train', 'val', 'test'):  # run normally
         run(**vars(opt))