[FIR] Fix registration of multiple compiler plugins

compiler/tests-common/tests/org/jetbrains/kotlin/fir/AbstractFirBaseDiagnosticsTest.kt

@@ -105,9 +105,6 @@ abstract class AbstractFirBaseDiagnosticsTest : BaseDiagnosticsTest() {
                 needRegisterJavaElementFinder = true
             ) {
                 configureSession()
-                getFirExtensions()?.let {
-                    registerExtensions(it)
-                }
             }
         }
 
@@ -127,10 +124,6 @@ abstract class AbstractFirBaseDiagnosticsTest : BaseDiagnosticsTest() {
         runAnalysis(testDataFile, files, firFilesPerSession)
     }
 
-    protected open fun getFirExtensions(): BunchOfRegisteredExtensions? {
-        return null
-    }
-
     private fun mapKtFilesToFirFiles(session: FirSession, ktFiles: List<KtFile>, firFiles: MutableList<FirFile>, useLightTree: Boolean) {
         val firProvider = (session.firProvider as FirProviderImpl)
         if (useLightTree) {