Remove unused indexes

Neither of these indexes have ever been scanned.

Author: sgrif

migrations/2019-06-14-195059_remove_unused_indexes/down.sql

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+CREATE INDEX index_version_num ON versions (num);
+CREATE INDEX path_categories_idx ON categories (path);

migrations/2019-06-14-195059_remove_unused_indexes/up.sql

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+DROP INDEX path_categories_idx;
+DROP INDEX index_version_num;