[K/JS Gradle] Remove forceLegacyBackendUsage from KotlinGradlePlugin previously added with the compiler flags refactoring, also fix bootstrap compilation of atomicfu plugin

compiler/testData/cli/js/legacyCompilerUsageWithoutFlag.args

@@ -0,0 +1,3 @@
+$TESTDATA_DIR$/simple2js.kt
+-output
+$TEMP_DIR$/out.js

compiler/testData/cli/js/legacyCompilerUsageWithoutFlag.out

@@ -0,0 +1,2 @@
+error: old Kotlin/JS compiler is no longer supported. Please migrate to the new JS IR backend
+COMPILATION_ERROR

compiler/tests-gen/org/jetbrains/kotlin/cli/CliTestGenerated.java

@@ -1363,6 +1363,11 @@ public class CliTestGenerated extends AbstractCliTest {
             runTest("compiler/testData/cli/js/languageVersion.args");
         }
 
+        @TestMetadata("legacyCompilerUsageWithoutFlag.args")
+        public void testLegacyCompilerUsageWithoutFlag() throws Exception {
+            runTest("compiler/testData/cli/js/legacyCompilerUsageWithoutFlag.args");
+        }
+
         @TestMetadata("libraryDirNotFound.args")
         public void testLibraryDirNotFound() throws Exception {
             runTest("compiler/testData/cli/js/libraryDirNotFound.args");