Remove import fallbacks.

src/transformers/tokenization_roberta.py

@@ -20,15 +20,6 @@ import logging
 from .tokenization_gpt2 import GPT2Tokenizer
 
 
-try:
-    from functools import lru_cache
-except ImportError:
-    # Just a dummy decorator to get the checks to run on python2
-    # because honestly I don't want to support a byte-level unicode BPE tokenizer on python 2 right now.
-    def lru_cache():
-        return lambda func: func
-
-
 logger = logging.getLogger(__name__)
 
 VOCAB_FILES_NAMES = {