Report error on -Xuse-old-backend, remove Gradle option useOldBackend

Allow using old JVM backend only to compile kts.

 #KT-48532 Fixed

compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/FE1FullPipelineModularizedTest.kt

@@ -6,15 +6,9 @@
 package org.jetbrains.kotlin.fir
 
 import org.jetbrains.kotlin.cli.common.arguments.K2JVMCompilerArguments
-import org.jetbrains.kotlin.cli.common.toBooleanLenient
-
-
-private val USE_BE_IR = System.getProperty("fir.bench.fe1.useIR", "false").toBooleanLenient()!!
 
 class FE1FullPipelineModularizedTest : AbstractFullPipelineModularizedTest() {
     override fun configureArguments(args: K2JVMCompilerArguments, moduleData: ModuleData) {
-        args.useIR = USE_BE_IR
-        args.useOldBackend = !USE_BE_IR
         args.useK2 = false
         args.jvmDefault = "compatibility"
         args.apiVersion = "1.4"
@@ -34,4 +28,4 @@ class FE1FullPipelineModularizedTest : AbstractFullPipelineModularizedTest() {
             runTestOnce(i)
         }
     }
-}
+}