cmd/compile: delete the runtime_internal_atomic.go builtin defs file

The file was automatically placed in the cl by a tool I had built.
Since the compiler doesn't hook into the atomic package, it's unnecessary.

Change-Id: I631fd876813b381bb12604865b00fc5b268dce84
Reviewed-on: https://go-review.googlesource.com/16844


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>