Move getCustomFieldTokens to the tokens class

This deprecates (quietly) another function on CRM_Utils_Token. It's a pretty small function -
the sort you'd just copy & paste if you wanted to use outside of core but
I didn't add noise. The only core place that calls this is the tokenTrait
- which is 'used' by activity tokens - but only for the handling of _N_
which I think was actually not supposed to have been included in the merge.

Anyway - I'm leaving that question for later because I'm not sure if the token
trait is used outside of core or how our final interface will look

CRM/Core/TokenTrait.php

@@ -84,7 +84,10 @@ public function getReturnFields(): array {
    */
   protected function getCustomFieldTokens(): array {
     if (!isset($this->customFieldTokens)) {
-      $this->customFieldTokens = \CRM_Utils_Token::getCustomFieldTokens(ucfirst($this->getEntityName()));
+      $this->customFieldTokens = [];
+      foreach (CRM_Core_BAO_CustomField::getFields(ucfirst($this->getEntityName())) as $id => $info) {
+        $this->customFieldTokens['custom_' . $id] = $info['label'] . ' :: ' . $info['groupTitle'];
+      }
     }
     return $this->customFieldTokens;
   }

CRM/Utils/Token.php

@@ -1523,6 +1523,11 @@ public static function getUserTokenReplacement($token, $escapeSmarty = FALSE) {
     return $value;
   }
 
+  /**
+   * Do not use.
+   *
+   * @deprecated
+   */
   protected static function _buildContributionTokens() {
     $key = 'contribution';
 
@@ -1544,6 +1549,10 @@ protected static function _buildContributionTokens() {
   }
 
   /**
+   * Do not use.
+   *
+   * @deprecated
+   *
    * Replace tokens for an entity.
    * @param string $entity
    * @param array $entityArray
@@ -1856,6 +1865,8 @@ public static function legacyContactTokens() {
   /**
    * Get all custom field tokens of $entity
    *
+   * @deprecated
+   *
    * @param string $entity
    * @return array
    *   return custom field tokens in array('custom_N' => 'label') format