magento/magento2#: Remove unused “Default Email Domain” option and related to it code

Author: atwixfirster

app/code/Magento/Customer/etc/adminhtml/system.xml

@@ -86,9 +86,6 @@
                     <comment>To show VAT number on Storefront, set Show VAT Number on Storefront option to Yes.</comment>
                     <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                 </field>
-                <field id="email_domain" translate="label" type="text" sortOrder="60" showInDefault="1" showInWebsite="1" showInStore="1" canRestore="1">
-                    <label>Default Email Domain</label>
-                </field>
                 <field id="email_template" translate="label comment" type="select" sortOrder="70" showInDefault="1" showInWebsite="1" showInStore="1" canRestore="1">
                     <label>Default Welcome Email</label>
                     <comment>Email template chosen based on theme fallback when "Default" option is selected.</comment>

app/code/Magento/Customer/etc/config.xml

@@ -15,7 +15,6 @@
                 <confirm>0</confirm>
                 <default_group>1</default_group>
                 <tax_calculation_address_type>billing</tax_calculation_address_type>
-                <email_domain>example.com</email_domain>
                 <email_identity>general</email_identity>
                 <email_template>customer_create_account_email_template</email_template>
                 <email_no_password_template>customer_create_account_email_no_password_template</email_no_password_template>

app/code/Magento/Sales/Model/AdminOrder/Create.php

@@ -29,7 +29,10 @@
 class Create extends \Magento\Framework\DataObject implements \Magento\Checkout\Model\Cart\CartInterface
 {
     /**
-     * Xml default email domain path
+     * Path to "CUSTOMERS > Customer Configuration > Create New Account Options > Default Email Domain" setting
+     *
+     * @deprecated since version 2.2.6
+     * @see \Magento\Sales\Model\AdminOrder\Create. Constant usage has been removed from _getNewCustomerEmail() method.
      */
     const XML_PATH_DEFAULT_EMAIL_DOMAIN = 'customer/create_account/email_domain';
 

app/code/Magento/Sales/etc/di.xml

@@ -1008,9 +1008,6 @@
                 <item name="sales_email/shipment/copy_to" xsi:type="string">1</item>
                 <item name="sales_email/shipment_comment/copy_to" xsi:type="string">1</item>
             </argument>
-            <argument name="environment" xsi:type="array">
-                <item name="customer/create_account/email_domain" xsi:type="string">1</item>
-            </argument>
         </arguments>
     </type>
     <preference