platform: Remove tornado.platform.auto.set_close_exec

platform: Remove tornado.platform.auto.set_close_exec

This function is obsolete: Since python 3.4, file descriptors created
by python are non-inheritable by default (and in the event you create
a file descriptor another way, a standard function os.set_inheritable
is available).

The windows implementation of this function was also apparently
broken, but this went unnoticed because the default behavior on
windows is for file descriptors to be non-inheritable.

Fixes #2867