[11.x] Remove a unused import and fix docblock for DeferredCallbackCollection #40785

Job	Run time
PHP 8.2 - PHPUnit 10.5 - prefer-lowest	2m 31s
PHP 8.2 - PHPUnit 10.5 - prefer-stable	2m 21s
PHP 8.2 - PHPUnit 10.5 - prefer-lowest - Windows	2m 22s
PHP 8.2 - PHPUnit 11.0.1 - prefer-lowest	2m 23s
PHP 8.2 - PHPUnit 11.0.1 - prefer-stable	2m 22s
PHP 8.2 - PHPUnit 10.5 - prefer-stable - Windows	2m 13s
PHP 8.3 - PHPUnit 10.5 - prefer-lowest	2m 2s
PHP 8.2 - PHPUnit 11.0.1 - prefer-lowest - Windows	2m 27s
PHP 8.3 - PHPUnit 10.5 - prefer-stable	1m 57s
PHP 8.3 - PHPUnit 11.0.1 - prefer-lowest	2m 2s
PHP 8.3 - PHPUnit 11.0.1 - prefer-stable	1m 59s
PHP 8.2 - PHPUnit 11.0.1 - prefer-stable - Windows	2m 16s
PHP 8.3 - PHPUnit 10.5 - prefer-lowest - Windows	2m 20s
PHP 8.3 - PHPUnit 10.5 - prefer-stable - Windows	2m 16s
PHP 8.3 - PHPUnit 11.0.1 - prefer-lowest - Windows	2m 20s
PHP 8.3 - PHPUnit 11.0.1 - prefer-stable - Windows	2m 17s
	36m 8s

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