Test always evaluates the same

Defect 114044 (STATIC_C)
Checker DEADCODE (subcategory redundant_test)
File: /src/btree/bt_curprev.c
Function __cursor_row_prev
/src/btree/bt_curprev.c, line: 555
At condition "upd != NULL", the value of "upd" cannot be "NULL".

            if (upd != NULL && upd->type == WT_UPDATE_TOMBSTONE) {

/src/btree/bt_curprev.c, line: 553
Condition "upd == NULL", taking false branch. Now the value of "upd" is not "NULL".

            if (upd == NULL)

/src/btree/bt_curprev.c, line: 555
The condition "upd != NULL" must be true.

            if (upd != NULL && upd->type == WT_UPDATE_TOMBSTONE) {