[Gradle, JS] Remove duplicate minimal Gradle version requirement

Minimal Gradle version for all Kotlin plugins is now 6.1 and minimal Gradle versions for all integration tests is set to 6.1.1

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kotlin2JsGradlePluginIT.kt

@@ -301,8 +301,6 @@ class Kotlin2JsGradlePluginIT : AbstractKotlin2JsGradlePluginIT(false) {
 }
 
 abstract class AbstractKotlin2JsGradlePluginIT(val irBackend: Boolean) : BaseGradleIT() {
-    override val defaultGradleVersion = GradleVersionRequired.AtLeast("6.1")
-
     override fun defaultBuildOptions(): BuildOptions =
         super.defaultBuildOptions().copy(
             jsIrBackend = irBackend,