Remove unit tests for duplicate-code detection

The old Semmle duplicate-code detection code has never been done when
extracting databases for the CodeQL CLI, except that `codeql test run`
will run it _just_ in order to support tests of the feature. With the
sunsetting of Odasa there's no need to even _test_ the feature anymore.

This commit removes those tests that fail when the duplicate-code
detector is turned off. Once it is merged and bumped, we can finally
remove it from `codeql`.

Author: Henning Makholm