Remove compiler variables

[ajschmidt8]

With the PR below merged, we no longer set the CXX, CC, or CUDAHOSTCXX variables in any of our CI images. This PR cleans up some references to them.

• Remove custom compilers gpuci-build-environment#265

[pentschev]

LGTM, thanks @ajschmidt8

[codecov-commenter]

Codecov Report

Merging #929 (aa211e1) into branch-22.08 (e2ff3d7) will not change coverage.
The diff coverage is n/a.

❗ Current head aa211e1 differs from pull request most recent head 78a1ac5. Consider uploading reports for the commit 78a1ac5 to get more accurate results

@@              Coverage Diff              @@
##           branch-22.08    #929    +/-   ##
=============================================
  Coverage          0.00%   0.00%            
=============================================
  Files                22      16     -6     
  Lines              3075    2094   -981     
=============================================
+ Misses             3075    2094   -981     

Impacted Files	Coverage Δ
dask_cuda/_version.py
dask_cuda/benchmarks/local_cudf_shuffle.py
dask_cuda/benchmarks/local_cupy_map_overlap.py
dask_cuda/benchmarks/utils.py
dask_cuda/benchmarks/local_cupy.py
dask_cuda/benchmarks/local_cudf_merge.py

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e2ff3d7...78a1ac5. Read the comment docs.

[ajschmidt8]

@gpucibot merge