Remove duplicate, failing oldest tests. (#7272)

Nightly tests failed last night at https://travis-ci.com/certbot/certbot/builds/120816454.

The cause was the oldest the version of Ubuntu used in the tests suddenly changed from Trusty to Xenial. You can see Xenial being used in the failing test at  https://travis-ci.com/certbot/certbot/jobs/219873088#L9 and Trusty being used at the last passing test at https://travis-ci.com/certbot/certbot/jobs/218936290#L9. The change in the default doesn't seem to be documented (yet) at https://docs.travis-ci.com/user/reference/overview/.

I started to pin Trusty in these tests, however, I noticed that we are running these same unit tests at e6bf3fe7f8/.travis.yml (L58). These other tests are still succeeding because it appears that including `sudo: required` causes Travis to still default to Trusty.

Deleting these duplicated tests fixes our Travis failures and speeds things up ever so slightly.

* Remove duplicate, failing oldest tests.

* pin trusty

.travis.yml

@@ -55,6 +55,10 @@ matrix:
       env: TOXENV=mypy
       <<: *not-on-master
     - python: "2.7"
+      # Ubuntu Trusty or older must be used because the oldest version of
+      # cryptography we support cannot be compiled against the version of
+      # OpenSSL in Xenial or newer.
+      dist: trusty
       env: TOXENV='py27-{acme,apache,certbot,dns,nginx}-oldest'
       sudo: required
       services: docker
@@ -132,12 +136,6 @@ matrix:
       sudo: required
       services: docker
       <<: *extended-test-suite
-    - python: "2.7"
-      env: TOXENV=py27-certbot-oldest
-      <<: *extended-test-suite
-    - python: "2.7"
-      env: TOXENV=py27-nginx-oldest
-      <<: *extended-test-suite
     - python: "2.7"
       env: ACME_SERVER=boulder-v1 TOXENV=integration-certbot-oldest
       sudo: required