Remove resowner_private.h

This header is not used since the refactoring of resource owners done in
b8bff07, and all the functions declared in it became (well, mostly)
static inline local to each resowner kind's code path.

Author: Xing Guo
Discussion: https://postgr.es/m/CACpMh+BFmtK5Z=b6PvH4HLKhUpWa_VtRTZSrB4-yK-tQejpWGw@mail.gmail.com

Author: michaelpq