Removed Default value options for Media Storage Location.

Added additional comment reminding of need to save settings
following synchronization of storage.

Author: Graham Wharton

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

@@ -10,11 +10,11 @@
         <section id="system" type="text" sortOrder="900" showInDefault="1" showInWebsite="1" showInStore="1">
             <group id="media_storage_configuration" translate="label" type="text" sortOrder="900" showInDefault="1" showInWebsite="1" showInStore="1">
                 <label>Storage Configuration for Media</label>
-                <field id="media_storage" translate="label" type="select" sortOrder="100" showInDefault="1" showInWebsite="0" showInStore="0" canRestore="1">
+                <field id="media_storage" translate="label" type="select" sortOrder="100" showInDefault="1" showInWebsite="0" showInStore="0">
                     <label>Media Storage</label>
                     <source_model>Magento\MediaStorage\Model\Config\Source\Storage\Media\Storage</source_model>
                 </field>
-                <field id="media_database" translate="label" type="select" sortOrder="200" showInDefault="1" showInWebsite="0" showInStore="0" canRestore="1">
+                <field id="media_database" translate="label" type="select" sortOrder="200" showInDefault="1" showInWebsite="0" showInStore="0">
                     <label>Select Media Database</label>
                     <source_model>Magento\MediaStorage\Model\Config\Source\Storage\Media\Database</source_model>
                     <backend_model>Magento\MediaStorage\Model\Config\Backend\Storage\Media\Database</backend_model>
@@ -24,7 +24,7 @@
                 </field>
                 <field id="synchronize" translate="label comment" type="button" sortOrder="300" showInDefault="1" showInWebsite="0" showInStore="0">
                     <frontend_model>Magento\MediaStorage\Block\System\Config\System\Storage\Media\Synchronize</frontend_model>
-                    <comment>After selecting a new media storage location, press the Synchronize button to transfer all media to that location. Media will not be available in the new location until the synchronization process is complete.</comment>
+                    <comment>After selecting a new media storage location, press the Synchronize button to transfer all media to that location. Media will not be available in the new location until the synchronization process is complete. Remember to "Save Config" following change of mode and synchronization.</comment>
                 </field>
                 <field id="configuration_update_time" translate="label" type="text" sortOrder="400" showInDefault="1" showInWebsite="0" showInStore="0" canRestore="1">
                     <label>Environment Update Time</label>