Remove pgstat_flush_wal()

Remove pgstat_flush_wal()

All the processes that generate WAL should call pgstat_report_wal() to
report all their statistics related to WAL, and this is already what
happens in the tree.  Keeping pgstat_report_wal() is confusing while the
other routine is encouraged.

This routine is not required since fc415edf8ca8, where it was lastly
used in pgstat_report_stat() before an equivalent callback existed.

Author: Bertrand Drouvot <[email protected]>
Discussion: https://postgr.es/m/[email protected]