Eliminate obsolete step parameter from doc2vec infer_vector and similarity_unseen_docs

[rock420]

Fix #3158

Eliminate the step parameter from the function infer_vector() , similarity_unseen_docs() and corresponding test function. (modification in the code and function documentation)
No other code/docs/examples using the outdated step parameter.

[gojomo]

Looks good to me.

Will raise new exceptions for user code using steps as parameter to infer_vector() or similarity_unseen_document() that was previously using the default silently.

As the previous behavior (to both accept steps and to ignore it) was buggy, I think that's OK in a 4.1 minor or even 4.0.x bugfix release, with proper mention in release notes – but other possible standards for what counts as a 'breaking change' might disagree.

[mpenkov]

Looks good. Thank you for your contribution!