mariadb-upgrade should remove mysql.plugin entries for plugins that became bundled

For example the unix_socket plugin was a loadable plugin up to 10.3, but became a compiled-in plugin starting with 10.4 due to the privilege tables structure changes. So if the plugin had e.g. be enabled with INSTALL PLUGIN in 10.3 then after an upgrade to 10.4 that mysql.plugin entry leads to a

[ERROR] mariadbd: Plugin 'unix_socket' is already installed.

log entry as the plugin is already active due to being built-in, and trying to load it once again while processing mysql.plugin entries will lead to that "already installed" notification.

As we know which plugins have been changed to be compiled in for a given release we should also take care of not triggering any warnings due to the version upgrade, these just confuse users as they didn't actually do anything wrong.

Or at a minimum the error message should be more clear on an attempt to load a plugin that's actually already built in into the current server binary, as at first sight I was assuming the usual "plugin_load_add vs INSTALL PLUGIN" situation, and doubted that the user had sent me the correct / complete configuration file ... so "plugin is already built in to the server" instead of "plugin is already loaded"