cmd/compile: remove -s flag

This is better handled by tools like cmd/gofmt, which can
automatically rewrite the source code and already supports a syntactic
version of this simplification. (go/types can be used if
type-sensitive simplification is actually necessary.)

Change-Id: I51332a8f3ff4ab3087bc6b43a491c6d92b717228
Reviewed-on: https://go-review.googlesource.com/c/go/+/197118


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>