Drop the hack needed to use CachingFileManager as we don't use it anymore. (#2544)

Author: alexamici

xarray/backends/cfgrib_.py

@@ -39,13 +39,6 @@ def __init__(self, filename, lock=None, **backend_kwargs):
         if lock is None:
             lock = ECCODES_LOCK
         self.lock = ensure_lock(lock)
-
-        # NOTE: filter_by_keys is a dict, but CachingFileManager only accepts
-        #   hashable types.
-        if 'filter_by_keys' in backend_kwargs:
-            filter_by_keys_items = backend_kwargs['filter_by_keys'].items()
-            backend_kwargs['filter_by_keys'] = tuple(filter_by_keys_items)
-
         self.ds = cfgrib.open_file(filename, **backend_kwargs)
 
     def open_store_variable(self, name, var):