fix JetPsiUtil.deleteClass() so that it no longer tries to delete the entire file when deleting a nested class or object

#KT-7483 Fixed

idea/testData/safeDelete/deleteObject/kotlinObject/companionObject.kt

@@ -0,0 +1,6 @@
+
+class A {
+    fun foo(): Int = 0
+
+    companion ob<caret>ject {}
+}

idea/testData/safeDelete/deleteObject/kotlinObject/companionObject.kt.after

@@ -0,0 +1,4 @@
+class A {
+    fun foo(): Int = 0
+
+}