Remove unused get_*.cmake files. (#57)

This PR removes some unused `get_*.cmake` files. I know cuVS may need to add some of these back as functionality is migrated from RAFT. As that happens, I would like to copy them again from the latest versions in RAFT.

Specifically, files like `get_thrust.cmake` are outdated (we are moving to `rapids_cpm_cccl` and `rapids_cpm_thrust` is deprecated) so this will reduce the maintenance burden for CMake / build system developers in the interim period.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Micka (https://github.com/lowener)

URL: #57

Author: bdice