Remove unneeded language feature configuration from fir:tree

It causes a verbose warning to be displayed on each build. Inline
classes are enabled by default since Kotlin 1.5, so this configuration
is no longer needed.

compiler/fir/tree/build.gradle.kts

@@ -56,9 +56,3 @@ if (kotlinBuildProperties.isInJpsBuildIdeaSync) {
         this.module.generatedSourceDirs.add(generationRoot)
     }
 }
-
-kotlin {
-    sourceSets.all {
-        languageSettings.enableLanguageFeature("InlineClasses")
-    }
-}