[llvm] [offload] Add early exit on plugin deinit when offloading is disabled (PR #134090)

[Michael Halkenhäuser via llvm-commits]

mhalk wrote:

> Apologies if my OMPT callback event comment caused confusion. I was not referring to downstream only functionality. I simply do not recall if the `init` and `deinit` parts cause OMPT callback events.

No, AFAICT no device-deinit is triggered, and consequently no OMPT (device) callback events show up.

>From my perspective it made sense to make the plugin `init` and `deinit` symmetrical.
(here: w.r.t. "do nothing", when offloading is disabled.)
This avoids the empty loop, as well as the debug prints.

https://github.com/llvm/llvm-project/pull/134090