Remove the DB index "note_mentions_temp_index"

The index note_mentions_temp_index on notes table was added for a feature that never rolled out. The index should be removed.

gitlabhq_production=> SELECT * FROM pg_stat_all_indexes WHERE indexrelname in ('note_mentions_temp_index');
 relid | indexrelid | schemaname | relname |       indexrelname       | idx_scan | idx_tup_read | idx_tup_fetch 
-------+------------+------------+---------+--------------------------+----------+--------------+---------------
 33614 | 3410300955 | public     | notes   | note_mentions_temp_index |        0 |            0 |             0
(1 row)