To install Spider one can simply drop a /etc/mysql/conf.d/spider.cnf like

  [mariadb]
  plugin-load-add=ha_spider.so

This is automatically generated and installed when plugin is correctly
registered to plugin.cmake with its own component name. Many other plugins
such as Connect and RocksDB install in the same way.

This solved MDEV-19917 as the mere adding and removing of spider.cnf
automatically installs and uninstalls it.

Remove the overly complex and uncecessary install.sql from Spider,
if should not be needed in modern times anymore.

With this change there is no need for a uninstall.sql either.