drop test_isl_functionality.cc test

It is not clear what these tests are checking.
Presumably, they were used to experiment with the isl C++ bindings,
but then they have served their purpose.
There is also some dead code (HasFilterChild) and commented out code
in there.

Author: Sven Verdoolaege

test/CMakeLists.txt

@@ -40,7 +40,6 @@ endif()
 set(CORE_TEST_FILES
   test_core
   test_inference
-  test_isl_functionality
   test_tc2halide
 )