Allow users to remove their emails if instance does not need email to register

lib/pleroma/user.ex

@@ -2248,7 +2248,7 @@ defmodule Pleroma.User do
   def change_email(user, email) do
     user
     |> cast(%{email: email}, [:email])
-    |> validate_required([:email])
+    |> maybe_validate_required_email(false)
     |> unique_constraint(:email)
     |> validate_format(:email, @email_regex)
     |> update_and_set_cache()

lib/pleroma/web/api_spec/operations/twitter_util_operation.ex

@@ -121,7 +121,10 @@ defmodule Pleroma.Web.ApiSpec.TwitterUtilOperation do
       type: :object,
       required: [:email, :password],
       properties: %{
-        email: %Schema{type: :string, description: "New email"},
+        email: %Schema{
+          type: :string,
+          description: "New email. Set to blank to remove the user's email."
+        },
         password: %Schema{type: :string, description: "Current password"}
       }
     }