Don't require to override abstract methods in expect class

#KT-16099 Fixed

compiler/testData/multiplatform/explicitActualOnOverrideOfAbstractMethod/common.kt

@@ -0,0 +1,5 @@
+expect abstract class Base {
+    abstract fun foo()
+}
+
+expect class DerivedImplicit : Base

compiler/testData/multiplatform/explicitActualOnOverrideOfAbstractMethod/jvm.kt

@@ -0,0 +1,7 @@
+actual abstract class Base {
+    actual abstract fun foo()
+}
+
+actual class DerivedImplicit : Base() {
+    override fun foo() {}
+}

compiler/testData/multiplatform/explicitActualOnOverrideOfAbstractMethod/output.txt

@@ -0,0 +1,7 @@
+-- Common --
+Exit code: OK
+Output:
+
+-- JVM --
+Exit code: OK
+Output: