Drop scala-collection-compat

We used to cross-build MiMa to 2.13, but that was pointless so we
dropped it (see #400).

Author: dwijnand

build.sbt

@@ -64,10 +64,7 @@ val functionalTests = Project("functional-tests", file("functional-tests"))
   .enablePlugins(TestsPlugin)
   .disablePlugins(BintrayPlugin)
   .settings(
-    libraryDependencies ++= Seq(
-      "com.typesafe"           %  "config"                  % "1.4.0",
-      "org.scala-lang.modules" %% "scala-collection-compat" % "2.1.3",
-    ),
+    libraryDependencies += "com.typesafe" % "config" % "1.4.0",
     mimaFailOnNoPrevious := false,
     skip in publish := true,
   )

functional-tests/src/main/scala/com/typesafe/tools/mima/lib/ProblemFiltersConfig.scala

@@ -3,7 +3,7 @@ package lib
 
 import com.typesafe.tools.mima.core.{ProblemFilter, ProblemFilters}
 import com.typesafe.config._
-import scala.jdk.CollectionConverters._
+import scala.collection.JavaConverters._
 
 object ProblemFiltersConfig {
   private val filterProblemsPath = "filter.problems"