[Libomptarget] Experimental Remote Plugin Fixes

D97883 introduced a compile-time error in the experimental remote offloading
libomptarget plugin, this patch fixes it and resolves a number of
inconsistencies in the plugin as well:

1. Non-functional Asynchronous API
2. Unnecessarily verbose debug printing
3. Misc. code clean ups

This is not intended to make any functional changes to the plugin.

Differential Revision: https://reviews.llvm.org/D105325