events: don't keep arrays with a single listener

Use the remaining listener directly if the array of listeners has only one element after running EventEmitter.prototype.removeListener().

Advantages:

• Better memory usage and better performance if no new listeners are added for the same event.

Disadvantages:

• A new array must be created if new listeners are added for the same event.

Checklist

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

Affected core subsystem(s)

events