pgsql: Simplify signature of CopyAttributeOutCSV() in copyto.c

Simplify signature of CopyAttributeOutCSV() in copyto.c

This has come up in 2889fd23be56, reverted later on, and is still useful
on its own to reduce a bit the differences between the code paths
dedicated to CSV and text.

Discussion: https://postgr.es/m/ZcCKwAeFrlOqPBuN@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b9d6038d7048ddb837aa1d286fb8f5ca0e9b3a83

Modified Files
--------------
src/backend/commands/copyto.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)