runtime: delete runtime/internal/atomic/textflag.h

As per mdempsky's comment on golang.org/cl/14204, textflag.h is
copied to the includes dir by cmd/dist, and the copy in
runtime/internal/atomic is not actually being used.

Updates #11647

Change-Id: Ie95c08903a9df54cea4c70ee9d5291176f7b5609
Reviewed-on: https://go-review.googlesource.com/16871
Run-TryBot: Michael Matloob <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: matloob