Revert "Deprecate "flex-require" sections"

This reverts commit c36d11d.

Author: nicolas-grekas

src/Flex.php

@@ -415,9 +415,6 @@ public function update(Event $event, $operations = [])
             if (!isset($json['flex-'.$type])) {
                 continue;
             }
-
-            $this->io->writeError(sprintf('<warning>Using section "flex-%s" in composer.json is deprecated, use "%1$s" instead.</>', $type));
-
             foreach ($json['flex-'.$type] as $package => $constraint) {
                 if ($symfonyVersion && '*' === $constraint && isset($versions['splits'][$package])) {
                     // replace unbounded constraints for symfony/* packages by extra.symfony.require