Fix mutable collection stub methods generation for corner case

The problem is that
`override fun remove(element: E): CollectionWithRemove<E>`
seems to be illegal from Java's point of view, while it's OK for JVM

These declarations have the same signature (return type is isgnored)
- override fun remove(element: E): CollectionWithRemove<E>
- override fun remove(element: E): Boolean

When we meet such declaration we choose random declaration for fake override in synthetic class
that may lead to signature clash

compiler/testData/codegen/box/builtinStubMethods/customReadOnlyIterator.kt

@@ -0,0 +1,34 @@
+class A : Collection<Char> {
+    override val size: Int
+        get() = throw UnsupportedOperationException()
+
+    override fun contains(element: Char): Boolean {
+        throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
+    }
+
+    override fun containsAll(elements: Collection<Char>): Boolean {
+        throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
+    }
+
+    override fun isEmpty(): Boolean {
+        throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.
+    }
+
+    override fun iterator() = MyIterator
+}
+
+object MyIterator : Iterator<Char> {
+    override fun hasNext() = true
+
+    override fun next() = 'a'
+}
+
+
+fun box(): String {
+    val it: MyIterator = A().iterator()
+
+    if (!it.hasNext()) return "fail 1"
+    if (it.next() != 'a') return "fail 2"
+
+    return "OK"
+}