Remove duplicate test file.

As noted in SciTools/iris#1244, `dec_subset.pp` occurs in both `test_data/PP/globClim1/` and `test_data/PP/model_comp`. The latter is only used once by tests, so the former copy is preferred.

There remains a single file in `test_data/PP/model_comp`, so I'm wondering if that should be moved to `globClim1` as well? The names of the second-level directories are not particularly descriptive...