all: cleanup build tags

For some cases, we already rely on Go1.9 as the minimally supported Go version,
in which case the build tag is always satisfied.

In other cases, we drop special casing for appengine or js. Instead, just rely
on the purego tag being properly specified in those specialized environments.

Change-Id: I8b1b1511ea85531166f0d778f0029bd41664075b
Reviewed-on: https://go-review.googlesource.com/c/151404
Reviewed-by: Herbie Ong <herbie@google.com>

proto/decode_test.go

@@ -29,8 +29,6 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build go1.7
-
 package proto_test
 
 import (

proto/encode_test.go

@@ -29,8 +29,6 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build go1.7
-
 package proto_test
 
 import (

proto/pointer_reflect.go

@@ -29,7 +29,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build purego appengine js
+// +build purego
 
 // This file contains an implementation of proto field accesses using package reflect.
 // It is slower than the code in pointer_unsafe.go but it avoids package unsafe and can

proto/pointer_unsafe.go

@@ -29,7 +29,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build !purego,!appengine,!js
+// +build !purego
 
 // This file contains the implementation of the proto field accesses using package unsafe.
 

protoc-gen-go/testdata/import_public_test.go

@@ -29,8 +29,6 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build go1.9
-
 package testdata
 
 import (