Restructure aggregatable_contributions table to avoid index on primary key

Rather than use the artificial contribution_id as the primary key, we
can use the pair of <aggregation_id, contribution_id> as a compound
primary key that allows for efficient lookups/deletions of contributions
by aggregation_id without the need for an index on the aggregation_id
column.

This is analogous to the optimization used by the dedup_keys table.

Change-Id: I77c25e05a74df880d64c022279b96384688f3afe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4139778
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Reviewed-by: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1089695}