Fix `clear_cache_on_start` in pystencils.json

Due two weird recursive dependencies `clear_cache` will not be
available at this point. In-lining this function will make
the config key `cache.clear_cache_on_start: true` work again.

pystencils/cpu/cpujit.py

@@ -194,7 +194,7 @@ def read_config():
         config['cache']['object_cache'] = os.path.expanduser(config['cache']['object_cache']).format(pid=os.getpid())
 
         if config['cache']['clear_cache_on_start']:
-            clear_cache()
+            shutil.rmtree(config['cache']['object_cache'], ignore_errors=True)
 
         create_folder(config['cache']['object_cache'], False)