Remove duplicated mish activation function (#7856)

* Remove duplicated mish activation function

* Update activations.py

src/transformers/activations.py

@@ -66,7 +66,3 @@ def get_activation(activation_string):
         return ACT2FN[activation_string]
     else:
         raise KeyError("function {} not found in ACT2FN mapping {}".format(activation_string, list(ACT2FN.keys())))
-
-
-def mish(x):
-    return x * torch.tanh(torch.nn.functional.softplus(x))