Revert "Update tokenizer.py"

This reverts commit 6f1b5d5.

With this commit active underscores weren't being correctly detokenized
as spaces.

https://community.libretranslate.com/t/the-problem-of-translating-symbols-oov-out-of-the-vocabulary/1071/4

argostranslate/tokenizer.py

@@ -24,7 +24,8 @@ def encode(self, sentence: str) -> List[str]:
         return tokens
 
     def decode(self, tokens: List[str]) -> str:
-        return self.lazy_processor().decode_pieces(tokens)
+        detokenized = "".join(tokens)
+        return detokenized.replace("▁", " ")
 
 
 class BPETokenizer(Tokenizer):