Remove `Conv._convolution_op` assignment in constructor.  This was kept as a temporary measure to preserve backwards compatibility.  Going forward `Conv` layer subclasses should prefer `Conv.convolution_op()`.
Remove `Conv._convolution_op` assignment in constructor.  This was kept as a temporary measure to preserve backwards compatibility.  Going forward `Conv` layer subclasses should prefer `Conv.convolution_op()`.

PiperOrigin-RevId: 388606987

keras-team/keras,keras-team/keras