Revert "Refactor CopyAttributeOut{CSV,Text}() to use a callback in COPY TO"

Revert "Refactor CopyAttributeOut{CSV,Text}() to use a callback in COPY TO"

This reverts commit 2889fd23be56, following a discussion with Andres
Freund as this callback, being called once per attribute when sending a
relation's row, can involve a lot of indirect function calls (more
attributes to deal with means more impact).  The effects of a dispatch
at this level would become more visible when improving the per-row code
execution of COPY TO, impacting future potential performance
improvements.

Discussion: https://postgr.es/m/20240206014125.qofww7ew3dx3v3uk@awork3.anarazel.de