Remove use of pg_memory_is_all_zeros() in bufpage.c
After a closer lookup, this makes the all-zero check of the page more
expensive, so let's remove the new function call in bufpage.c. The
maths of the check were also incorrect, checking that the page was full
of zeros only for the first 1kB.
This brings back the code to the state it was at
49d6c7d8daba.
Per discussion with David Rowley and Bertrand Drouvot.
Discussion: https://postgr.es/m/CAApHDvrXzPAr3FxoBuB7b3D-okNoNA2jxLun1rW8Yw5wkbqusw@mail.gmail.com