fix Coroutine\Barrier mem leak

[Appla]

the static property $cancel_list hold the value set in __destruct in some case.
after barrier object destructed by set it's reference by null, we can unset this that value added in __destruct

the following test case could trigger this issue

    /**
     * Test without execution switching between coroutines.
     *
     * @covers \Swoole\Coroutine\Barrier
     */
    public function testNoCoroutineSwitching()
    {
        run(function () {
            $barrier = Barrier::make();
            $count = 0;
            $N = 4;
            foreach (range(1, $N) as $i) {
                \Swoole\Coroutine::create(function () use ($barrier, &$count) {
                    $count++;
                });
            }
            Barrier::wait($barrier);

            $this->assertSame($N, $count, 'The parent coroutine keeps running without switching execution to child coroutines.');
        });
    }

[deminy]

Linking to a similar PR (#69) submitted by @sy-records .

[deminy]

Thanks for the PR. My only suggestion is that if there are multiple improvements/bug fixes to address, please consider separating them into different PRs, and each focuses on a specific issue.