ci: Travis: remove non-coverage jobs

This helps with regard to slowness until
pytest-dev#6470 is resolved.

.travis.yml

@@ -1,9 +1,5 @@
 language: python
 dist: xenial
-stages:
-- baseline
-- test
-
 python: '3.7'
 cache: false
 
@@ -46,25 +42,11 @@ jobs:
     - env: TOXENV=py37-lsof-numpy-oldattrs-pexpect-twisted PYTEST_COVERAGE=1 PYTEST_ADDOPTS=
       python: '3.7'
 
-    # Coverage tracking is slow with pypy, skip it.
-    - env: TOXENV=pypy3-xdist
-      python: 'pypy3'
-
     # Coverage for Python 3.5.{0,1} specific code, mostly typing related.
     - env: TOXENV=py35 PYTEST_COVERAGE=1 PYTEST_ADDOPTS="-k test_raises_cyclic_reference"
       python: '3.5.1'
       dist: trusty
 
-    # Specialized factors for py37.
-    - env: TOXENV=py37-pluggymaster-xdist
-    - env: TOXENV=py37-freeze
-
-    - env: TOXENV=py38-xdist
-      python: '3.8'
-
-    - stage: baseline
-      env: TOXENV=py36-xdist
-      python: '3.6'
     - env: TOXENV=linting,docs,doctesting PYTEST_COVERAGE=1
       cache:
         directories: