[K/JS] Remove an internal system property and replace it with the new compiler flag

compiler/testData/integration/ant/js/bothPrefixAndPostfix/build.log.expected

@@ -3,14 +3,6 @@ Buildfile: [TestData]/build.xml
 
 build:
 [kotlin2js] Compiling [[TestData]/root1] => [[Temp]/out.js]
-[kotlin2js] warning: ==========
-[kotlin2js] This project currently uses the Kotlin/JS Legacy compiler backend, which has been deprecated and will be removed in a future release.
-[kotlin2js]
-[kotlin2js] Please migrate your project to the new IR-based compiler (https://kotl.in/jsir).
-[kotlin2js] Because your build tool will not support the new Kotlin/JS compiler, you will also need to migrate to Gradle.
-[kotlin2js]
-[kotlin2js] You can continue to use the deprecated legacy compiler in the current version of the toolchain by providing the compiler option -Xuse-deprecated-legacy-compiler.
-[kotlin2js] ==========
 
 BUILD SUCCESSFUL
 Total time: [time]

compiler/testData/integration/ant/js/bothPrefixAndPostfix/build.xml

@@ -6,7 +6,7 @@
                output="${temp}/out.js"
                outputPrefix="${test.data}/prefix"
                outputPostfix="${test.data}/postfix">
-      <compilerarg value="-Xuse-deprecated-legacy-compiler"/>
+      <compilerarg value="-Xforce-deprecated-legacy-compiler-usage"/>
     </kotlin2js>
   </target>
 </project>