In commit 2f6df937
we fixed an observed case of the bug by removing
some code related to the no longer needed
BTR_MODIFY_PREV mode.

In commit 73ad436e
an alternative fix was applied that also fixes the
BTR_SEARCH_PREV case.

Let us clean up some implicit references to BTR_MODIFY_PREV
that were missed in 2f6df937.

btr_pcur_move_backward_from_page(): Assume that the latch mode was
BTR_SEARCH_LEAF.

btr_pcur_move_to_prev(): Assert that the latch mode is BTR_SEARCH_LEAF.
This function is mostly invoked in row0sel.cc for read operations,
as well as in row0merge.cc for reading from the clustered index.
All callers indeed use a cursor in the BTR_SEARCH_LEAF mode.