don't vary scala version in CI matrix

rather, varying the Scala version is handled by MultiScalaProject

Author: SethTisue

.travis.yml

@@ -4,12 +4,6 @@ import: scala/scala-dev:travis/default.yml
 
 language: scala
 
-scala:
-  - 2.11.12
-  - 2.12.15
-  - 2.13.6
-  - 3.0.2
-
 env:
   - SCALAJS_VERSION=          ADOPTOPENJDK=8
   - SCALAJS_VERSION=1.7.0     ADOPTOPENJDK=8