!9400 Delete redundant phrase in LGamma doc

From: @peixu_ren
Reviewed-by: @liangchenghui,@zh_qh
Signed-off-by: @zh_qh

mindspore/nn/layer/math.py

@@ -247,9 +247,6 @@ class LGamma(Cell):
     Outputs:
         Tensor, has the same shape and dtype as the `x`.
 
-    Supported Platforms:
-        ``Ascend``
-
     Examples:
         >>> input_x = Tensor(np.array([2, 3, 4]).astype(np.float32))
         >>> op = nn.LGamma()