Remove Travis-CI configuration (#7674)

We've moved to Github CI exclusively, because Travis has unfortunately
become exceedingly slow.  Remove the obsolete configuration file.

.travis.yml

@@ -1,71 +0,0 @@
-# TravisCI left in repo as a backup CI solution in case GitHub CI suffers a
-# major disruption.  Only a few, quick tests are run in order to keep the
-# TravisCI runtime equal to the GitHub CI time (to avoid bottlenecking on
-# TravisCI).
-#
-# If GitHub CI goes away, it would make sense to drop this .YML file and
-# use the complete one from release tag 2.7.2
-
-
-language: bash
-os: linux
-dist: bionic
-
-git:
-  depth: 1
-  submodules: false
-
-before_install:
-  - git submodule update --init  # no recursive update
-
-stages:
-  - build
-  - deploy
-
-
-# Run only 5 jobs since TravisCI only allows 5 in parallel
-jobs:
-  include:
-    - name: "Platform.IO"
-      stage: build
-      script: $TRAVIS_BUILD_DIR/tests/platformio.sh
-      install:
-          - sudo apt-get install python3-pip python3-setuptools
-      env:
-        # PIO is very slow, so do 1/2 as many builds as Arduino
-        - BUILD_PARITY=custom mod=20 rem=5
-
-    - name: "Build 1/4"
-      stage: build
-      script: $TRAVIS_BUILD_DIR/tests/build.sh
-      env:
-        - BUILD_PARITY=custom mod=10 rem=1
-
-    - name: "Build 2/4"
-      stage: build
-      script: $TRAVIS_BUILD_DIR/tests/build.sh
-      env:
-        - BUILD_PARITY=custom mod=10 rem=2
-
-    - name: "Build 3/4"
-      stage: build
-      script: $TRAVIS_BUILD_DIR/tests/build.sh
-      env:
-        - BUILD_PARITY=custom mod=10 rem=3
-
-    - name: "Build 4/4"
-      stage: build
-      script: $TRAVIS_BUILD_DIR/tests/build.sh
-      env:
-        - BUILD_PARITY=custom mod=10 rem=4
-
-notifications:
-  email:
-    on_success: change
-    on_failure: change
-  webhooks:
-    urls:
-      - secure: "dnSY+KA7NK+KD+Z71copmANDUsyVePrZ0iXvXxmqMEQv+lp3j2Z87G5pHn7j0WNcNZrejJqOdbElJ9Q4QESRaAYxTR7cA6ameJeEKHiFJrQtN/4abvoXb9E1CxpL8aNON/xgnqCk+fycOK3nbWWXlJBodzBm7KN64vrcHO7et+M="
-    on_success: change  # options: [always|never|change] default: always
-    on_failure: always  # options: [always|never|change] default: always
-    on_start: false     # default: false