backport MDEV-26912 Spider: Remove dead code related to Oracle OCI

The oracle and oracle2 test suites have been disabled for a long time and not run by any CI builders, and keeping them will simply add confusion. And it's always good to delete dead code which proved to be fine (MDEV-26912 was closed more than two years ago).

Other reasons:

• The feature is disabled by default, and there is no documentation in my knowledge on how to build with this feature included
• the last time anyone made any changes to storage/spider/spd_db_oracle.cc that is not part of a wider refactoring* is commit 982290fe9b88811fa184659469fc4381528a6a9e by Kentoku on Thu May 13 01:51:19 2021 +0900. Since then there have been many changes to the more general spider code, with changes specific to the mariadb backend code. I suspect the absence of corresponding oracle oci backend code changes probably means even if the code worked back then, it probably won't work now
• *: an exception was MDEV-28992, but then again, I did not run the oracle tests