API Remove deprecated extension hooks

We don't really have a way to deprecate these per-se, but they were
removed in other admin sections already so it's time to remove them from
here as well.

code/LeftAndMain.php

@@ -45,7 +45,6 @@
 use SilverStripe\Model\ArrayData;
 use SilverStripe\View\Requirements;
 use SilverStripe\View\SSViewer;
-use SilverStripe\Dev\Deprecation;
 
 /**
  * LeftAndMain is the parent class of all the two-pane views in the CMS.

code/ModalController.php

@@ -10,7 +10,6 @@
 use SilverStripe\Control\HTTPResponse;
 use SilverStripe\Core\Injector\Injector;
 use SilverStripe\Forms\Form;
-use SilverStripe\Dev\Deprecation;
 
 /**
  * Parent controller for all CMS-global modals

code/ModelAdmin.php

@@ -321,19 +321,6 @@ protected function getGridFieldConfig(): GridFieldConfig
             ->addComponent($exportButton)
             ->addComponents(Injector::inst()->createWithArgs(GridFieldPrintButton::class, ['buttons-before-left']));
 
-        // Remove default and add our own filter header with extension points
-        // to retain API until deprecation in 5.0
-        $config->removeComponentsByType(GridFieldFilterHeader::class);
-        $config->addComponent(Injector::inst()->createWithArgs(GridFieldFilterHeader::class, [
-            false,
-            function ($context) {
-                $this->extend('updateSearchContext', $context);
-            },
-            function ($form) {
-                $this->extend('updateSearchForm', $form);
-            }
-        ]));
-
         if (!$this->showSearchForm ||
             (is_array($this->showSearchForm) && !in_array($this->getModelClass(), $this->showSearchForm ?? []))
         ) {