Follow up from matomo-org/matomo-package#112

It's been a while since we released Matomo 4 meaning we can now remove Provider and CustomVariables plugin from the core release and only ship it only through the marketplace.

We no longer want to have in matomo.zip release zip the directories plugins/Providers and plugins/CustomVariables. Of course we also need to test that this won't cause any issues for people that are already on Matomo 4.