Remove unnecessary #ifdefs and dead code, in Spider, which is protected by #ifdef.
Some time ago, the Spider was a separate project, and Kentoku tried to make it compile on many MySQL and MariaDB versions without changes. So, there are many #ifdef macro in the Spider codebase, which were to adjust Spider for changes in API over different server versions. For example, see ITEM_FUNC_CASE_PARAMS_ARE_PUBLIC. Now, the Spider is part of the source tree, and thus we can remove dead code in #ifdef.
Use unifdef as far as possible.