Remove deprecated mxnet guppy
Currently a DeprecationWarning is raised whenever any guppy is imported, if mxnet is installed. This is not the intended behavior as it should be only raised if an mxnet guppy is actually used. Since the mxnet guppy is deprecated since a long time, it should just be removed. Any fish model that might use mxnet or any other framework should be implemented in the fish_models repo.