CI: Remove notifications comming from Travis CI (#663)

With enough people using GitHub directly and using PR for code submission,
there is no need to send the emails to the mailing list or group of developers.
Travis status shows right next to each commit and PR and together with results from GitHub Actions.

.travis.yml

@@ -37,12 +37,4 @@ after_success:
   - bash < (curl -s https://codecov.io/bash)
 
 notifications:
-        email:
-            recipients:
-                - grass-dev@lists.osgeo.org
-                - landa.martin@gmail.com
-                - wenzeslaus@gmail.com
-            on_success: change
-            on_failure: always
         irc: "chat.freenode.net#grass"
-