fix inconsistencies in `return_all_coefficients` across inference methods

[9q9q]

Default used to be False but changed to "none" recently for some, should standardize across all inference methods. Also, at least one method (IHT) does not use the arg even though it is specified, so remove.

[9q9q]

Oops, just realized the new options are only for LCA, so no need to change the others which still default to False. But will remove the unused arg.