Merge pgstat_count_io_op_n() and pgstat_count_io_op()

Merge pgstat_count_io_op_n() and pgstat_count_io_op()

The pgstat_count_io_op() function, which counts a single I/O operation,
wraps pgstat_count_io_op_n() with a counter value of 1.  The latter is
declared in pgstat.h and used nowhere in the code, so let's remove it in
favor of the former.

This change makes also the code more symmetric with
pgstat_count_io_op_time(), that already uses a similar set of arguments,
except that it counts also the I/O time.  This will ease a bit the
integration of a follow-up patch that adds byte-level tracking in
pg_stat_io for some of its attributes, lifting the current restriction
based on BLCKSZ as all I/O operations are assumed to be block-based.

Author: Nazir Bilal Yavuz
Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/CAN55FZ32ze812=yjyZg1QeXhKvACUM_Nu0_gyPQcUKKuVHL5xA@mail.gmail.com