Remove seemingly invalid SoftPlus (apache#7189)

- `Softplus` is added in 12/10/2020 from this apache#7089
- However, I see that there were `SoftPlus` (not the P is in capital) was already in.
According to [Onnx spec](https://github.com/onnx/onnx/blob/master/docs/Operators.md), it is `Softplus` not `SoftPlus`.

python/tvm/relay/frontend/onnx.py

@@ -932,14 +932,6 @@ def _impl_v1(cls, inputs, attr, params):
         return _op.tanh(_expr.const(beta) * inputs[0]) * _expr.const(alpha)
 
 
-class SoftPlus(OnnxOpConverter):
-    """Operator converter for SoftPlus."""
-
-    @classmethod
-    def _impl_v1(cls, inputs, attr, params):
-        return _op.log(_op.exp(inputs[0]) + _expr.const(1.0))
-
-
 class Softsign(OnnxOpConverter):
     """Operator converter for Softsign."""
 
@@ -2661,7 +2653,6 @@ def _get_convert_map(opset):
         "OneHot": OneHot.get_converter(opset),
         # 'Hardmax'
         "Softsign": Softsign.get_converter(opset),
-        "SoftPlus": SoftPlus.get_converter(opset),
         "Gemm": Gemm.get_converter(opset),
         "MatMul": MatMul.get_converter(opset),
         "Mod": Mod.get_converter(opset),

tests/python/frontend/onnx/test_forward.py

@@ -1983,7 +1983,6 @@ def verify_single_ops(op, x, out_np, rtol=1e-5, atol=1e-5):
     verify_single_ops("Tanh", x, np.tanh(x))
     verify_single_ops("Sigmoid", x, 1 / (1 + np.exp(-x)))
     verify_single_ops("Softsign", x, x / (1 + np.abs(x)))
-    verify_single_ops("SoftPlus", x, np.log(1 + np.exp(x)))
 
 
 @tvm.testing.uses_gpu