Improve output buffering when used for request_filesystem_credentials()

[ocean90]

wp_ajax_delete_theme(), wp_ajax_delete_plugin(), delete_theme() and delete_plugins() wrap request_filesystem_credentials() in ob_start() and ob_end_clean(). But the later is used in a condition which can lead to unexpected results since it's not always called.

[ocean90]

In 38167:

Filesystem API: Output buffering for request_filesystem_credentials() should wrap the function directly.

Previously ob_end_clean() was only called when the previous condition was successful which led to unexpected results when another output buffering was involved, like PHPUnit's.

Fixes #37488.