Drop the general index on activities.

Indices should be more specific as the general index can lead to
slower queries because postgres can't estimate the specificity of
jsonb indexes very well.

priv/repo/migrations/20170620095947_remove_activities_index.exs

+defmodule Pleroma.Repo.Migrations.RemoveActivitiesIndex do
+  use Ecto.Migration
+
+  def change do
+    drop index(:activities, [:data])
+  end
+end