Drop cudnn from CUDA 12 migration

Since `cudnn` is no longer in `zip_keys` with `cuda_compiler_version`
and the version of `cudnn` used by CUDA 12 is the same as CUDA 11, go
ahead and drop `cudnn` from the CUDA 12 migrator. It is not actually
needed or used here.

recipe/migrations/cuda120.yaml

@@ -77,9 +77,6 @@ cxx_compiler_version:          # [linux and os.environ.get("CF_CUDA_ENABLED", "F
 fortran_compiler_version:      # [linux and os.environ.get("CF_CUDA_ENABLED", "False") == "True"]
   - 12                         # [linux and os.environ.get("CF_CUDA_ENABLED", "False") == "True"]
 
-cudnn:                         # [linux and os.environ.get("CF_CUDA_ENABLED", "False") == "True"]
-  - 8                          # [linux and os.environ.get("CF_CUDA_ENABLED", "False") == "True"]
-
 cdt_name:                      # [linux and os.environ.get("CF_CUDA_ENABLED", "False") == "True"]
   - cos7                       # [linux and os.environ.get("CF_CUDA_ENABLED", "False") == "True"]