and translate to python if possible"

Enables to simply not have such an example key, instead of being
forced to provide an empty one.

... to prevent messing up more complex ones.

5260683 (DOC: use dataset argument in Python example if CMD example
has it too) added a `dataset` argument to several Python examples that
use a bound dataset method.  This would raise an exception: the
dataset is already being specified through the bound method.

As discussed in dataladgh-4091 [*], it might make sense to use unbound
methods consistently in all Python API examples, but until will make
that switch, remove the `dataset` argument from these spots.

[*]: datalad#4091 (comment)