Remove single-burst special case, keep phase linking results to `phas… e487ce2 …e_linking/`

[scottstanie]

Currently the multi-burst outputs have one directory per burst ID, while the single-burst is a special case that moves those contents to the top level work_directory. This is unnecessary, and has led to problems such as https://github.com/opera-adt/disp-s1/issues/296

This always places the phase linking results in one directory up from work, naming it phase_linking in case it's a general non-OPERA swath.