Partially backport #32217 (#32225)

Remove unnecessary `_getCustomClass` method.

`customClass` will be in the configuration already, even if it is provided in the data attributes, and jQuery's [`addClass()`](https://api.jquery.com/addclass/) method will handle it, either `this.config.customClass` is a function or a class name (even if it is a blank string).

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

js/src/tooltip.js

@@ -286,7 +286,7 @@ class Tooltip {
       this._popper = new Popper(this.element, tip, this._getPopperConfig(attachment))
 
       $(tip).addClass(CLASS_NAME_SHOW)
-      $(tip).addClass(this._getCustomClass())
+      $(tip).addClass(this.config.customClass)
 
       // If this is a touch-enabled device we add extra
       // empty mouseover listeners to the body's immediate children;
@@ -734,10 +734,6 @@ class Tooltip {
     this.config.animation = initConfigAnimation
   }
 
-  _getCustomClass() {
-    return this.element.getAttribute('data-custom-class') || this.config.customClass
-  }
-
   // Static
 
   static _jQueryInterface(config) {