package/ci: drop Gitter webhook notifications.

It never really worked, was either too verbose or too quiet, and now it
404s on AppVeyor.

package/ci/appveyor.yml

@@ -77,13 +77,6 @@ environment:
     APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
     APPVEYOR_JOB_NAME: windows-rt-gles3-msvc2017
 
-notifications:
-- provider: Webhook
-  url: https://webhooks.gitter.im/e/415ae90928ba0dbd3df4
-  on_build_success: false
-  on_build_failure: true
-  on_build_status_changed: true
-
 install:
 # Ninja. `cinst ninja` started 503ing in late November 2019 and wasn't really
 # reliable before either. So install by hand, as that's reliable always.

package/ci/circleci.yml

@@ -3,12 +3,6 @@ version: 2.1
 orbs:
   codecov: codecov/codecov@1.1.1
 
-notify:
-  webhooks:
-    # Unfortunately Gitter messages are too verbose (one five-line message per
-    # each job in the build matrix) and thus not wanted
-    #- url: https://webhooks.gitter.im/e/cfbadbd34d28708a57c6
-
 executors:
   ubuntu-16_04:
     docker:

package/ci/travis.yml

@@ -258,14 +258,6 @@ matrix:
       - android-22
       - sys-img-armeabi-v7a-android-22
 
-notifications:
-  webhooks:
-    urls:
-      - https://webhooks.gitter.im/e/00ec7a9066bb0fec98f3
-    on_success: change
-    on_failure: always
-    on_start: never
-
 cache:
   directories:
   - $HOME/sdl2