Remove 'spack bootstrap' and associated docs (#15179)

fixes #15145

This commit removes the outdated `spack bootstrap`
command and any reference to it in the documentation
and unit tests.

.travis.yml

@@ -22,11 +22,6 @@ jobs:
       os: linux
       language: python
       env: TEST_SUITE=flake8
-# Shell integration with module files
-    - python: '3.8'
-      os: linux
-      language: python
-      env: [ TEST_SUITE=bootstrap ]
     - stage: 'unit tests + documentation'
       python: '2.6'
       dist: trusty