Remove unused spline kernel.inc files

[krystophny]

Problem

Three unused include files exist in src/interpolate/:

src/interpolate/spline1d_many_point_body_kernel.inc
src/interpolate/spline2d_many_point_body_kernel.inc
src/interpolate/spline3d_many_point_body_kernel.inc

These are not referenced anywhere in the codebase. The actual files in use are:

src/interpolate/spline1d_many_point_body.inc
src/interpolate/spline2d_many_point_body.inc
src/interpolate/spline3d_many_point_body.inc

Evidence

$ grep -r "kernel.inc" src/interpolate/
# No matches

$ grep -r "point_body.inc" src/interpolate/
batch_interpolate_1d.f90:549:            include "spline1d_many_point_body.inc"
batch_interpolate_1d.f90:581:            include "spline1d_many_point_body.inc"
# ... (6 total references to point_body.inc files)

Solution

Delete the unused files:

rm src/interpolate/spline{1,2,3}d_many_point_body_kernel.inc

Impact

Cleanup only - no functional change.