Debloat by removing duplicate metabib index entries

Debloat by removing duplicate metabib index entries

The bloaty indexer added rows like so:

 id  | source | field |                                value
-----+--------+-------+----------------------------------------------------------------------
 178 |     47 |     5 | Concertos, piano
 179 |     47 |     5 | Rapsodie sur un thème de Paganini, piano, orchestra
 180 |     47 |     5 | Concertos, piano Rapsodie sur un thème de Paganini, piano, orchestra

Remove all but the last row for every matching source/field rows.

Signed-off-by: Dan Scott <dscott@laurentian.ca>