added a config schema update the settings form and add a update hook to remove the the unused config settings

config/schema/upei_roblib_ill.schema.yml

@@ -0,0 +1,28 @@
+upei_roblib_ill.settings:
+  type: config_object
+  label: 'UPEI Roblib ILL settings'
+  mapping:
+    ill_library_symbol:
+      type: string
+      label: 'Your ILL Library Symbol'
+    ill_doi_openurl_pid:
+      type: string
+      label: 'OpenURL PID'
+    ill_contact_email:
+      type: string
+      label: 'Contact Email'
+    ill_contact_phone:
+      type: string
+      label: 'Contact Phone Number'
+    rapid_ill_username:
+      type: string
+      label: 'RapidILL Username'
+    rapid_ill_password:
+      type: string
+      label: 'RapidILL Password'
+    rapid_ill_code:
+      type: string
+      label: 'RapidILL Rapid Code'
+    rapid_ill_branch_name:
+      type: string
+      label: 'RapidILL Branch Name'

src/Form/RoblibIllSettingsForm.php

@@ -121,10 +121,7 @@ class RoblibIllSettingsForm extends FormBase
   public function submitForm(array &$form, FormStateInterface $form_state)
   {
     $config = \Drupal::configFactory()->getEditable('upei_roblib_ill.settings');
-    $config->set('ill_add_url', $form_state->getValue('ill_add_url'))->save();
-    $config->set('ill_auth_url', $form_state->getValue('ill_auth_url'))->save();
     $config->set('ill_library_symbol', $form_state->getValue('ill_library_symbol'))->save();
-    $config->set('ill_relais_key', $form_state->getValue('ill_relais_key'))->save();
     $config->set('ill_doi_openurl_pid', $form_state->getValue('ill_doi_openurl_pid'))->save();
     $config->set('ill_contact_email', $form_state->getValue('ill_contact_email'))->save();
     $config->set('ill_contact_phone', $form_state->getValue('ill_contact_phone'))->save();

upei_roblib_ill.install

@@ -16,6 +16,17 @@ function upei_roblib_ill_update_7100() {
   );
 }
 
+/**
+ * Remove obsolete Relais configuration keys.
+ */
+function upei_roblib_ill_update_9301() {
+  \Drupal::configFactory()->getEditable('upei_roblib_ill.settings')
+    ->clear('ill_add_url')
+    ->clear('ill_auth_url')
+    ->clear('ill_relais_key')
+    ->save();
+}
+
 /**
  * Implements hook_schema().
  */