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
PR-URL: #12840
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

BUILDING.md

@@ -124,15 +124,6 @@ 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: