Remove optix plugin

[iche033]

see discussion in #1199 (comment)

The optix plugin hasn't received any activities for many years. Our CIs do not build the optix plugin or run its tests. There are also no foreseeable plans to continue working on this plugin. So The decision is to remove it completely.

[iche033]

there was a discussion during PMC meeting to move the optix plugin code to a separate repo but eventually we decided to just add info in somewhere in readme / changelog / migration guide to point to a branch with the optix code.