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