Remove assertion in pgstat_count_io_op()
An equivalent check is done with pgstat_is_ioop_tracked_in_bytes(), so
there is no need for this extra one. Small cleanup that should have
been included in
f92c854cf406.
Author: Nazir Bilal Yavuz
Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/CAN55FZ0oqxBaaHAEsj=xFqkzE3n5P=3RA1V_igXwL-RV7QRzyw@mail.gmail.com