BTR_MODIFY_PREV: Remove. This mode was only used by the change buffer,
which commit f27e9c89 (MDEV-29694)
removed.

buf_page_get_gen(): Revert the change that was made in
commit 90b95c61 (MDEV-33543)
because it is not applicable after MDEV-29694. This fixes the
performance regression that Vladislav Vaintroub reported.

This is a 11.x specific fix; this needs to be fixed differently
in older major versions where the change buffer is present.