[Libomptarget] Remove the remote and ve plugins from libomptarget

These plugins are unmaintained and are not in a workable state. The VE
plugin has not been touched for years and has never had any running
tests. The remote plugin is in an unfinished state and is not production
ready upstream. These will need to be ported to the new nextgen
interface in the future if they are needed.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D154548