doc/progs: remove flaky timing-sensitive test

Package time already has enough inherently flaky tests covering its
behavior.  No need for more of them.

Fixes #10632.

Change-Id: I1229e9fcc2e28ba2c9b0b79f73638e35dbbe8bbf
Reviewed-on: https://go-review.googlesource.com/9517


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>