remove outdated code in conv3d

[MARD1NO]

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[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

PyTorch resnet50 time: 138.8ms (= 6938.5ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 126.4ms (= 6320.7ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
Relative speed: 1.10 (= 138.8ms / 126.4ms)

PyTorch resnet50 time: 84.0ms (= 4201.1ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 74.1ms (= 3704.8ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
Relative speed: 1.13 (= 84.0ms / 74.1ms)

PyTorch resnet50 time: 58.1ms (= 2903.5ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 47.3ms (= 2366.1ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
Relative speed: 1.23 (= 58.1ms / 47.3ms)

PyTorch resnet50 time: 43.5ms (= 2174.4ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 41.8ms (= 2092.3ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
Relative speed: 1.04 (= 43.5ms / 41.8ms)

PyTorch resnet50 time: 45.1ms (= 2256.3ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 46.3ms (= 2312.7ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
Relative speed: 0.98 (= 45.1ms / 46.3ms)