[Linux] remove AtkText stub implementation (flutter#34839)

When Orca has "Speak object under mouse" enabled, it tries to get the
text range extents for any object that implements the AtkText interface
and gets a bit confused by our AtkText stub implementation that merely
returns null for `get_text`.

Removing the stub implementation helps to avoid the issue that Orca
would try to call `atk_text_get_range_extents()` with `start_offset=0`
and `end_offset=0`:

> Atk-CRITICAL **: atk_text_get_range_extents: assertion 'start_offset >= 0 && start_offset < end_offset' failed

shell/platform/linux/fl_accessible_node.cc

@@ -89,7 +89,6 @@ enum { kProp0, kPropEngine, kPropId, kPropLast };
 static void fl_accessible_node_component_interface_init(
     AtkComponentIface* iface);
 static void fl_accessible_node_action_interface_init(AtkActionIface* iface);
-static void fl_accessible_node_text_interface_init(AtkTextIface* iface);
 
 G_DEFINE_TYPE_WITH_CODE(
     FlAccessibleNode,
@@ -99,9 +98,7 @@ G_DEFINE_TYPE_WITH_CODE(
         G_IMPLEMENT_INTERFACE(ATK_TYPE_COMPONENT,
                               fl_accessible_node_component_interface_init)
             G_IMPLEMENT_INTERFACE(ATK_TYPE_ACTION,
-                                  fl_accessible_node_action_interface_init)
-                G_IMPLEMENT_INTERFACE(ATK_TYPE_TEXT,
-                                      fl_accessible_node_text_interface_init))
+                                  fl_accessible_node_action_interface_init))
 
 // Returns TRUE if [flag] has changed between [old_flags] and [flags].
 static gboolean flag_is_changed(FlutterSemanticsFlag old_flags,
@@ -338,13 +335,6 @@ static const gchar* fl_accessible_node_get_name(AtkAction* action, gint i) {
   return data->name;
 }
 
-// Implements AtkText::get_text.
-static gchar* fl_accessible_node_get_text(AtkText* text,
-                                          gint start_offset,
-                                          gint end_offset) {
-  return nullptr;
-}
-
 // Implements FlAccessibleNode::set_name.
 static void fl_accessible_node_set_name_impl(FlAccessibleNode* self,
                                              const gchar* name) {
@@ -475,10 +465,6 @@ static void fl_accessible_node_action_interface_init(AtkActionIface* iface) {
   iface->get_name = fl_accessible_node_get_name;
 }
 
-static void fl_accessible_node_text_interface_init(AtkTextIface* iface) {
-  iface->get_text = fl_accessible_node_get_text;
-}
-
 static void fl_accessible_node_init(FlAccessibleNode* self) {
   FlAccessibleNodePrivate* priv = FL_ACCESSIBLE_NODE_GET_PRIVATE(self);
   priv->actions = g_ptr_array_new();