src: fix `size` underflow in CallbackQueue

Only decrease the size when actually removing items.

PR-URL: https://github.com/nodejs/node/pull/34662
Fixes: https://github.com/nodejs/node/issues/34657
Refs: https://github.com/nodejs/node/pull/34572


Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: James M Snell <jasnell@gmail.com>