Tests: Remove outdated portion of CompileFeatures genex test

The genex part of the test verifies that `$<COMPILE_FEATURES:...>`
evaluates as expected.  It does not need to actually try using code with
the associated features, as that is tested separately.

Tests/CompileFeatures/genex_test.cpp

@@ -31,17 +31,6 @@
 #  if !EXPECT_OVERRIDE_CONTROL
 #    error "Expect no override control feature"
 #  endif
-
-struct A
-{
-  virtual int getA() { return 7; }
-};
-
-struct B final : A
-{
-  int getA() override { return 42; }
-};
-
 #endif
 
 #if !HAVE_AUTO_TYPE