JVM IR: do not generate generic signature for $delegate field

See ee7bbbf530

compiler/testData/writeSignature/backingFieldForGenericDelegated.kt

@@ -1,5 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
-
 import kotlin.reflect.KProperty
 import kotlin.reflect.KProperty1
 
@@ -17,4 +15,4 @@ class Test {
 
 // field: Test::additionalText$delegate
 // jvm signature: LDVal;
-// generic signature: null
+// generic signature: null