pgsql: 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 <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z71oPkJJICrRB5Ws@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d7cbeaf261da346d8c745870da1d31075ae0313c

Modified Files
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src/backend/utils/activity/pgstat_wal.c | 11 +----------
src/include/utils/pgstat_internal.h     |  2 --
2 files changed, 1 insertion(+), 12 deletions(-)