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.

Refs: #12836

Author: Trott

BUILDING.md

@@ -123,15 +123,6 @@ To run the tests:
 $ make test
 ```
 
-To run the npm test suite:
-
-*note: to run the suite on node v4 or earlier you must first*
-*run `make install`*
-
-```console
-$ make test-npm
-```
-
 To build the documentation:
 
 This will build Node.js first (if necessary) and then use it to build the docs: