Remove unused functions

The following functions are not used in `structcol`:
- `_trans_coeffs()`
- `_time_coeffs()`
- `_W0()`
- `_scat_fields_complex_medium()`
- `diff_abs_intensity_complex_medium()`
- `calc_energy()`
- `calc_dwell_time()`
- `calc_reflectance()`

Should we delete or deprecate?