Remove fmgr.h includes from headers that don't really need it.
Most of the fmgr.h includes were obsoleted by
352a24a1f9d6f7d4abb1. A
few others can be obsoleted using the underlying struct type in an
implementation detail.
Author: Andres Freund
Discussion: https://postgr.es/m/
20190803193733.g3l3x3o42uv4qj7l@alap3.anarazel.de