[LMDB] fix uninitialized variable.

initialize mc_flags as it is used/read in mdb_page_search_root().

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
[LMDB] remove logically dead code

The MDB_PREV_DUP is already tested in the if()

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>