Drop "appenginevm" tag.

"appengine" and "appenginevm" are mutually exclusive.
diff --git a/proto/pointer_unsafe.go b/proto/pointer_unsafe.go
index c52db1c..48bc0fa 100644
--- a/proto/pointer_unsafe.go
+++ b/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 !appengine appenginevm
+// +build !appengine
 
 // This file contains the implementation of the proto field accesses using package unsafe.