Remove other uses of /Zc:__cplusplus in the documentation.

[rcurtin]

This is just a follow-up to #3555, since that option is no longer required for MSVC.

[conradsnicta]

There's a small fly in the ointment. Perhaps merge this only just before (or just after) the release of mlpack 4.2.2.

mlpack 4.2.1 does not have the updated C++17 detection under MSVC. If people download mlpack 4.2.1 (from mlpack.org) and then follow the updated instructions (doc/user/build_windows.md and doc/user/sample_ml_app.md), mlpack 4.2.1 is not going to work for them under MSVC.

Right now the current documentation works for both mlpack 4.2.1 and the git master branch. Use of the _MSVC_LANG macro (in #3555) can be thought of as a workaround for the bizarre and confusing config options to enable C++17 under MSVC. The workaround is there to make people's life easier if and when they confused by MSVC weirdness. Suggest to keep the current documentation to cover the changeover period between mlpack 4.2.1 and 4.2.2.

[rcurtin]

Yes, true, although the documentation distributed with 4.2.1 will be accurate. (It is only inaccurate if a user comes to Github to read documentation, which is probably the most common use case realistically like you pointed out.) I don't mind releasing 4.2.2 immediately after merging this, it's easy enough and minimizes the period of inaccurate documentation for older versions.

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍

[rcurtin]

#3547 is so close that I'm going to wait for that to be merged to merge this, and then open a release PR.