Remove default column from services table

What does this MR do?

After removing and ignoring the default column from the code in !38203 (merged), we can now drop the database column.

Migration script output

$ rails db:migrate
== 20200819082334 RemoveDefaultFromServices: migrating ========================
-- remove_column(:services, :default, :boolean)
   -> 0.0012s
== 20200819082334 RemoveDefaultFromServices: migrated (0.0013s) ===============

$ rails db:migrate:down VERSION=20200819082334
== 20200819082334 RemoveDefaultFromServices: reverting ========================
-- add_column(:services, :default, :boolean, {:default=>false})
   -> 0.0015s
== 20200819082334 RemoveDefaultFromServices: reverted (0.0042s) ===============

Related to #217581 (closed)