doc: remove test-npm from general build doc

make test-npm is not particularly robust (currently fails on Linux and Windows, reportedly) and results in a fair number of requests for help from people new to the project. It is used when upgrading npm in core, which only a small number of mostly-predefined people do. Remove it from the general build doc.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc