[OpenMP][NFC] Remove unused and untested code from the device runtime

Summary:
We carried a lot of unused and untested code in the device runtime.
Among other reasons, we are planning major rewrites for which reduced
size is going to help a lot.

The number of code lines reduced by 14%!

Before:
-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
CUDA                            13            489            841           2454
C/C++ Header                    14            322            493           1377
C                               12            117            124            559
CMake                            4             64             64            262
C++                              1              6              6             39
-------------------------------------------------------------------------------
SUM:                            44            998           1528           4691
-------------------------------------------------------------------------------

After:
-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
CUDA                            13            366            733           1879
C/C++ Header                    14            317            484           1293
C                               12            117            124            559
CMake                            4             64             64            262
C++                              1              6              6             39
-------------------------------------------------------------------------------
SUM:                            44            870           1411           4032
-------------------------------------------------------------------------------

Reviewers: hfinkel, jhuber6, fghanim, JonChesterfield, grokos, AndreyChurbanov, ye-luo, tianshilei1992, ggeorgakoudis, Hahnfeld, ABataev, hbae, ronlieb, gregrodgers

Subscribers: jvesely, yaxunl, bollu, guansong, jfb, sstefan1, aaron.ballman, openmp-commits, cfe-commits

Tags: #clang, #openmp

Differential Revision: https://reviews.llvm.org/D83349