Delete use stub from asm.s, leaving only a dummy file.
Deleting the file causes Windows build to fail.

Fixes #16607

Change-Id: Ic5a55e042e588f1e1bc6605a3d309d1eabdeb288
Reviewed-on: https://go-review.googlesource.com/36716Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>