Replace redundant `x_shape`
Description
The new DensityEstimators will have a _condition_shape attribute, which is required to correctly handle batch shapes on x_o.
Currently, all posterior classes also have a _x_shape attribute which does contain the same information and is thus no longer required.
@manuelgloeckler has this been fixed by #1151 ?
This is actually solved: in the posterior docstrings we say it's deprecated and if it is passed by a user we issue a deprecation warning. We can leave as it is for now and remove the argument from all the posterior classes in the next release.
Great! We should also check that the trainers and potentials also do no longer have x_shape