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.