console: delete timers that have ended

Currently, console timers that have been ended with timeEnd()
are not removed. This has the potential to leak memory. This
commit deletes ended timers from the containing Map.

PR-URL: https://github.com/nodejs/node/pull/3562


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>