Clean up redundant -Xmulti-platform option from multiplatform projects

It was necessary before, but now it's provided automatically.

libraries/kotlin.test/js/build.gradle

@@ -12,8 +12,7 @@ dependencies {
 }
 
 compileKotlin2Js {
-    // TODO: Why "-Xmulti-platfrom" ?
-    kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package", "-Xmulti-platform"]
+    kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package"]
     kotlinOptions {
         moduleKind = "umd"
         outputFile = "${buildDir}/classes/main/kotlin-test.js"

libraries/kotlin.test/junit/build.gradle

@@ -29,7 +29,7 @@ dist {
 }
 
 compileKotlin {
-    kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package", "-Xmulti-platform", "-module-name", project.name]
+    kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package", "-module-name", project.name]
 }
 
 compileTestKotlin {

libraries/kotlin.test/jvm/build.gradle

@@ -32,9 +32,8 @@ dist {
 }
 
 compileKotlin {
-    // TODO: Why "-Xmulti-platfrom" ?
     kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package", "-Xnormalize-constructor-calls=enable",
-                                      "-Xmulti-platform", "-module-name", project.archivesBaseName]
+                                      "-module-name", project.archivesBaseName]
 }
 
 compileTestKotlin {

libraries/kotlin.test/testng/build.gradle

@@ -29,7 +29,7 @@ dist {
 }
 
 compileKotlin {
-    kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package", "-Xmulti-platform", "-module-name", project.name]
+    kotlinOptions.freeCompilerArgs = ["-Xallow-kotlin-package", "-module-name", project.name]
 }
 
 compileTestKotlin {