perf_hooks: remove docs for unimplemented API

The node frame (aka loop) timing API did not land, it depends on
https://github.com/libuv/libuv/pull/1489 which is still a WIP.

See: https://github.com/nodejs/node/pull/14680#discussion_r140605664

PR-URL: https://github.com/nodejs/node/pull/15641


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>