db-manager: simplify signatures query
db-manager: simplify signatures query

This query was really slow cause the join was on a column
that wasn't indexed. After examining the usage of this func,
we only need the columns from pull_signatures anyway.

It went from 1400ms to < 10ms.

iFixit/pulldasher,iFixit/pulldasher,iFixit/pulldasher,iFixit/pulldasher