uninstall_plugin should do an include_once not include

[pcfreak30]

This seems to be an edge case, but it is possible for a plugin file to already be loaded when uninstall_plugin is run (like if done programmatically).

This can cause a fatal, and the simple solution is to use include_once.

[valchovski]

Hey!

Updated both include calls to include_once instead in the uninstall_plugin function

[SergeyBiryukov]

In 48326:

Plugins: Use include_once instead of include in uninstall_plugin(), in case the plugin file is already included.

Props valchovski, pcfreak30.
Fixes #47796.