K2/Native: simplify forward declarations test to make it work with K2

kotlin-native/backend.native/tests/interop/forwardDeclarations/forwardDeclarations.kt

@@ -1,9 +1,7 @@
 // This test mostly checks frontend behaviour.
 
 import cForwardDeclarations.*
-
-// Note: there are no practical cases for this type, but it works in K1 so better to have a test for this.
-import cnames.structs.StructUndeclared
+import cnames.structs.StructDeclared
 
 import kotlin.test.assertEquals
 import kotlinx.cinterop.COpaque
@@ -11,32 +9,22 @@ import kotlinx.cinterop.CStructVar
 import kotlinx.cinterop.ptr
 // The test should also check that these references can't be resolved, but the test infra doesn't support this yet:
 // import cForwardDeclarations.StructUndeclared
+// import cnames.structs.StructUndeclared // Supported in K1 though.
 
 fun <T1 : T2, T2> checkSubtype2() {}
-fun <T1 : T2, T2 : T3, T3> checkSubtype3() {}
-fun <T1 : T2, T2 : T3, T3 : T4, T4> checkSubtype4() {}
 
 // Here we rely on frontend reporting conflicting overloads if some of these types turn out to be the same.
-fun checkDifferentTypes(s: StructUndeclared?) = 0
 fun checkDifferentTypes(s: StructDeclared?) = 1
 fun checkDifferentTypes(s: StructDefined?) = 2
 
 fun main() {
-    // Checking it is the same type:
-    checkSubtype3<StructUndeclared, cnames.structs.StructUndeclared, StructUndeclared>()
-    checkSubtype2<StructUndeclared, COpaque>()
-
-    checkSubtype4<StructDeclared, cnames.structs.StructDeclared, cForwardDeclarations.StructDeclared, StructDeclared>()
     checkSubtype2<StructDeclared, COpaque>()
 
-    checkSubtype4<StructDefined, cnames.structs.StructDefined, cForwardDeclarations.StructDefined, StructDefined>()
     checkSubtype2<StructDefined, CStructVar>()
 
-    val undeclared: StructUndeclared? = null
     val declared: StructDeclared? = null
     val defined: StructDefined? = null
 
-    assertEquals(0, checkDifferentTypes(undeclared))
     assertEquals(1, checkDifferentTypes(declared))
     assertEquals(2, checkDifferentTypes(defined))