db: remove tag_aliases.post_count column.

This was only used in autocomplete, but it was unnecessary here because
we could already get the post count from the tags table.

db/migrate/20191117200404_drop_post_count_from_tag_aliases.rb

@@ -0,0 +1,6 @@
+class DropPostCountFromTagAliases < ActiveRecord::Migration[6.0]
+  def change
+    remove_index :tag_aliases, :post_count
+    remove_column :tag_aliases, :post_count, :integer, null: false, default: 0
+  end
+end