fix: remove VASP pseudopotential linking from CP2K FP workflow (#1802)

CP2K workflows were incorrectly calling VASP-style pseudopotential
linking functions, causing `KeyError: 'fp_pp_path'` when these
VASP-specific parameters weren't provided.

## Problem

The `make_fp_cp2k` function was unconditionally calling
`_link_fp_vasp_pp()`, which expects `fp_pp_path` and `fp_pp_files`
parameters. However, CP2K manages pseudopotentials and basis sets
through:
- Directives in CP2K input templates 
- Environment variables (e.g., `CP2K_DATA_DIR`)
- Built-in pseudopotential libraries

CP2K does not use VASP-style symbolic linking of pseudopotential files.

## Solution

Completely removed the `_link_fp_vasp_pp()` call from `make_fp_cp2k`.
This change:
- ✅ Eliminates the KeyError when `fp_pp_path`/`fp_pp_files` are not
provided
- ✅ Properly decouples CP2K from VASP-style pseudopotential handling
- ✅ Aligns with CP2K's native workflow requirements
- ✅ Maintains all existing CP2K functionality

## Testing

All existing tests pass, including CP2K-specific tests. The
`param-ch4.json` example now works correctly without requiring any
pseudopotential configuration parameters.

Fixes #1754.

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: njzjz <[email protected]>

Author: Copilot

dpgen/generator/run.py

@@ -3791,9 +3791,6 @@ def make_fp_cp2k(iter_index, jdata):
             fp.close()
         os.chdir(cwd)
 
-    # link pp files
-    _link_fp_vasp_pp(iter_index, jdata)
-
 
 def make_fp_pwmat(iter_index, jdata):
     # abs path for fp_incar if it exists