Remove redundant index on taggings table

What does this MR do and why?

Remove a redundant index on taggings, index_taggings_on_taggable_id_and_taggable_type defined over (taggable_id, taggable_type). This table already has another index with the same leading columns index_taggings_on_taggable_id_and_taggable_type_and_context defined over (taggable_id, taggable_type, context).

We can get a nice savings from this:

gitlabhq_production=> \di+ index_taggings_on_taggable_id_and_taggable_type
                                             List of relations
 Schema |                      Name                       | Type  | Owner  |  Table   | Size  | Description
--------+-------------------------------------------------+-------+--------+----------+-------+-------------
 public | index_taggings_on_taggable_id_and_taggable_type | index | gitlab | taggings | 32 GB |

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