plugin: load plugins with unknown license strings

We shouldn't fail to load plugins just because we don't
recognise the license string. It's not our job to validate
licenses, and the license list is outdated and ambiguous

Part-of: <gstreamer/gstreamer!865>
34 jobs for !865 with backport-mr-864-into-1.18-plugin-loading-license-check in 35 minutes and 1 second (queued for 3 seconds)
merge request