[FIR] Remove FirDeclarationsForMetadataProviderExtension extension

`IrGeneratedDeclarationsRegistrar` should be now used instead to register
  IR declarations in resulting metadata

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/extensions/FirExtensionRegistrar.kt

@@ -34,7 +34,6 @@ abstract class FirExtensionRegistrar : FirExtensionRegistrarAdapter() {
             FirScriptConfiguratorExtension::class,
             Fir2IrScriptConfiguratorExtension::class,
             FirFunctionTypeKindExtension::class,
-            FirDeclarationsForMetadataProviderExtension::class,
         )
 
         internal val ALLOWED_EXTENSIONS_FOR_LIBRARY_SESSION = listOf(
@@ -108,11 +107,6 @@ abstract class FirExtensionRegistrar : FirExtensionRegistrarAdapter() {
             registerExtension(FirFunctionTypeKindExtension::class, this)
         }
 
-        @JvmName("plusDeclarationForMetadataProviderExtension")
-        operator fun (FirDeclarationsForMetadataProviderExtension.Factory).unaryPlus() {
-            registerExtension(FirDeclarationsForMetadataProviderExtension::class, this)
-        }
-
         // ------------------ reference methods ------------------
 
         @JvmName("plusStatusTransformerExtension")
@@ -175,11 +169,6 @@ abstract class FirExtensionRegistrar : FirExtensionRegistrarAdapter() {
             FirFunctionTypeKindExtension.Factory { this.invoke(it) }.unaryPlus()
         }
 
-        @JvmName("plusDeclarationForMetadataProviderExtension")
-        operator fun ((FirSession) -> FirDeclarationsForMetadataProviderExtension).unaryPlus() {
-            FirDeclarationsForMetadataProviderExtension.Factory { this.invoke(it) }.unaryPlus()
-        }
-
         // ------------------ utilities ------------------
 
         @JvmName("bindLeft")