Cleanup and bugfixes for some 3-body pair styles in the GPU package

[ndtrung81]

Summary

The changes in this PR involve removing unused variables, some as addressed by #2197, more cleanup, and fixing bugs with building the short neighbor list in tersoff/gpu, tersoff/mod/gpu and tersoff/zbl/gpu that are triggered with multiple atom types.

Related Issues

The small but significant difference in the results between the GPU runs in double precision and the CPU-only run for tersoff/gpu with multiple atom types is first pointed out by @akohlmey when testing the changes in #2197.

Author(s)

Trung Nguyen (Northwestern)

Licensing

By submitting this pull request, I agree, that my contribution will be included in LAMMPS and redistributed under either the GNU General Public License version 2 (GPL v2) or the GNU Lesser General Public License version 2.1 (LGPL v2.1).

Backward Compatibility

Should maintain backward compability

Implementation Notes

The short neighbor lists are built with differently for the 3-body pair styles (tersoff/*, vashishta and sw).

Post Submission Checklist

• The feature or features in this pull request is complete
• Licensing information is complete
• Corresponding author information is complete
• The source code follows the LAMMPS formatting guidelines
• The feature has been verified to work with the conventional build system