The problem was that UNINSTALL PLUGIN wasn't performing privilege
checks before removing a plugin. Any user (including users without 
any kind of privileges) could uninstall any plugin.

The solution is to verify if the user has the DELETE privilege for
the mysql.plugin table before uninstalling a plugin.

mysql-test/r/plugin_not_embedded.result:
  Add test case result for Bug#51770.
mysql-test/t/plugin_not_embedded-master.opt:
  Add example plugin path.
mysql-test/t/plugin_not_embedded.test:
  Add test case for Bug#51770.
  Skip embedded as test relies on privileges checks.