pgsql: bufmgr: Remove the, now obsolete, BM_JUST_DIRTIED

bufmgr: Remove the, now obsolete, BM_JUST_DIRTIED

Due to the recent changes to use a share-exclusive mode for setting hint bits
and for flushing pages - instead of using share mode as before - a buffer
cannot be dirtied while the flush is ongoing.  The reason we needed
JUST_DIRTIED was to handle the case where the buffer was dirtied while IO was
ongoing - which is not possible anymore.

Discussion: https://postgr.es/m/5ubipyssiju5twkb7zgqwdr7q2vhpkpmuelxfpanetlk6ofnop@hvxb4g2amb2d

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b0f4ff3c92664e28615e2f885195c803156d47eb

Modified Files
--------------
src/backend/storage/buffer/bufmgr.c   | 30 +++++++++---------------------
src/backend/storage/buffer/localbuf.c |  2 +-
src/include/storage/buf_internals.h   |  3 +--
3 files changed, 11 insertions(+), 24 deletions(-)