pg_recvlogical: remove unnecessary OutputFsync() return value checks.

pg_recvlogical: remove unnecessary OutputFsync() return value checks.

Commit 1e2fddfa33d changed OutputFsync() so that it always returns true.
However, pg_recvlogical.c still contained checks of its boolean return
value, which are now redundant.

This commit removes those checks and changes the type of return value of
OutputFsync() to void, simplifying the code.

Suggested-by: Yilin Zhang <jiezhilove@126.com>
Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Mircea Cadariu <cadariu.mircea@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwFeTymZQ7RLvMU6WuDGar8bUQCazg=VOfA-9GeBkg-FzA@mail.gmail.com