runtime: remove walltime function

There was only one meaningful caller, which changes to call time_now.

This clearly separates systems that use walltime1 to be just those
that use the stub version of time_now. That is to say, those that do
not provide an assembler version of time_now.

Change-Id: I14c06cc402070bd705f953af6f9966785015e2a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/314769


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>