cmd/compile: remove encoding of safemode bit from export data

Removes the encoding of this bit which was ignored but left behind
for 1.7 to minimize pre-1.7 export format changes. See the issue
for more details.

Fixes #15772.

Change-Id: I46cd7a66ad4c6003b78c64295cf3bda503ebf2dd
Reviewed-on: https://go-review.googlesource.com/27201


Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>