[libomptarget] Remove unused image from global data movement function

This interface function does not actually need the device image type.
It's unused in the function, so it should be able to be safely removed.
The motivation for this is to facilitate downsteam porting of the
amd-stg-open RPC module into the nextgen plugin so we can delete the old
plugin entirely. For that to work we need to be able to call this
function at kernel-launch time, which doesn't have the image. Also it's
cleaner.

Reviewed By: jplehr

Differential Revision: https://reviews.llvm.org/D144436