Clean up "crimson_hexagon" from topic platform source pairs

[pypt]

Does adding select insert_platform_source_pair( 'twitter', 'brandwatch' ); here in mediawords.sql entail a migration, and should I also delete select insert_platform_source_pair( 'twitter', 'crimson_hexagon' );?

I think it does - that function preloads the table with some platform-source pairs, so you might want to DELETE from the table that it preloads into to get rid of that deprecated platform-source pair.

...and all of the rows in other tables with reference that particular row in topic_sources.

Related: #793