C++: Delete library-tests/qlcfg/cfg.ql

This test was intended to catch regressions in the CFG, but it looks
like it's just catching insignificant extractor changes. The test has
started failing after some recent extractor changes, but I have no way
to pinpoint the failure and understand whether it's a problem or not, so
I think it's better to delete this test.

The remaining tests check whether the QL-based CFG generates the same
graph as the extractor-based CFG. Furthermore, the `successor-tests`
check that the extractor-based CFG works as intended.

Author: jbj