diagnostics_channel: fix diagnostics channel memory leak

fix diagnostics channel memory leak. cc @Qard

const { subscribe, unsubscribe } = require('diagnostics_channel');

function noop() {}

console.log(process.memoryUsage().heapUsed);

for (let i = 0; i < 1000000; i++) {
    subscribe(String(i), noop);
    unsubscribe(String(i), noop);
}

gc();

console.log(process.memoryUsage().heapUsed);

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines