reference: remove useless var reset

cppcheck:

src/util-reference-config.c:179:9: warning: Assignment of function parameter has no effect outside the function. Did you forget dereferencing it? [uselessAssignmentPtrArg]
        fd = NULL;
        ^

Bug: #5291.

src/util-reference-config.c

@@ -176,7 +176,6 @@ static void SCRConfDeInitLocalResources(DetectEngineCtx *de_ctx, FILE *fd)
 {
     if (fd != NULL) {
         fclose(fd);
-        fd = NULL;
     }
 
     return;