tests/instance-id: remove unneeded local variable

The local `fd` shadows the class member `fd` causing a resource leak.

Change-Id: I20d55f7c56244e14741988f3322e01a8aeb5267b
Signed-off-by: Rashmica Gupta <rashmica@linux.ibm.com>
diff --git a/tests/instance-id.cpp b/tests/instance-id.cpp
index 57b6df2..3a4e89b 100644
--- a/tests/instance-id.cpp
+++ b/tests/instance-id.cpp
@@ -40,7 +40,6 @@
     {
         static const char dbTmpl[] = "db.XXXXXX";
         char dbName[sizeof(dbTmpl)] = {};
-        int fd;
 
         ::strncpy(dbName, dbTmpl, sizeof(dbName));
         fd = ::mkstemp(dbName);