Remove an unused file

The file plugin_exports became unused in
commit fec844ac

storage/innobase/plugin_exports

-{
- global:
-   _maria_plugin_interface_version_;
-   _maria_sizeof_struct_st_plugin_;
-   _maria_plugin_declarations_;
-   my_snprintf_service;
-   thd_alloc_service;
-   thd_autoinc_service;
-   thd_error_context_service;
-   thd_kill_statement_service;
-   thd_wait_service;
- local:
-   *;
-};