pgsql: Remove unneeded argument from _bt_getstackbuf().

Remove unneeded argument from _bt_getstackbuf().

_bt_getstackbuf() is called at exactly two points following commit
efada2b8e92 (one call site is concerned with page splits, while the
other is concerned with page deletion). The parent buffer returned by
_bt_getstackbuf() is write-locked in both cases. Remove the 'access'
argument and make _bt_getstackbuf() assume that callers require a
write-lock.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2ab23445bc6af517bddb40d8429146d8ff8d7ff4

Modified Files
--------------
src/backend/access/nbtree/nbtinsert.c | 11 ++++++-----
src/backend/access/nbtree/nbtpage.c | 2 +-
src/include/access/nbtree.h | 2 +-
3 files changed, 8 insertions(+), 7 deletions(-)