drop slapos.recipe.build:npm

This is not used, not tested and causes SyntaxWarning on python 3.8:

    slapos/recipe/npm.py:128: SyntaxWarning: "is not" with a literal. Did you mean "!="?
      if npm_process.returncode is not 0:
6 jobs for feat/git-2.38.1 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:01

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:43

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:59

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:45

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:04

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:18