[Libomptarget] Remove remaining global constructors in plugins (#75814)

Summary:
This patch fixes the remaining global constructor in the plguins after
addressing the ones in the JIT interface. This struct was mistakenly
using global constructors as not all the members were being initialized
properly. This was almost certainly being optimized out because it's
trivial, but would still be present in debug builds and prevented us
from compiling with `-Werror=global-constructors`. We will want to do
that once offloading is moved to a runtimes only build.