[Docs] Remove outdated docs on CUDA_DEVICE_ORDER and CUDA_VISIBLE_DEVICES

[SkafteNicki]

What does this PR do?

Fixes #4924
Removes section in docs saying that lightning sets CUDA_DEVICE_ORDER and CUDA_VISIBLE_DEVICES for the user, which we do not do anymore.

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[codecov]

Codecov Report

Merging #5287 (3afb530) into master (9ebbfec) will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5287   +/-   ##
======================================
  Coverage      93%     93%           
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  Files         134     134           
  Lines        9950    9945    -5     
======================================
- Hits         9261    9259    -2     
+ Misses        689     686    -3