x/mobile/internal/mobileinit: revert Android log prefix

CL 11981 changed the Android logcat prefix from "GoLog" to "GoStdio".
Revert the prefix to "GoLog".

Also remove a global variable left unused by CL 11981.

Change-Id: I500313652cd618d8573640d5165e5069a0ba0ba7
Reviewed-on: https://go-review.googlesource.com/11991
Reviewed-by: David Crawshaw <crawshaw@golang.org>

internal/mobileinit/mobileinit_android.go

@@ -31,8 +31,7 @@ import (
 )
 
 var (
-	ctag    = C.CString("GoStdio")
-	ctagLog = C.CString("GoLog")
+	ctag = C.CString("GoLog")
 )
 
 type infoWriter struct{}