util: remove unused variable inside virFileReadValueString().

The commit 69b937f0 introduced VIR_AUTOFREE and this macro removed
VIR_FREE. This change showed that 'str' variable was not being used
inside this method. This commit removes this unused variable.
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>
Reviewed-by: NRoman Bogorodskiy <bogorodskiy@gmail.com>

src/util/virfile.c

@@ -4228,7 +4228,6 @@ int
 virFileReadValueString(char **value, const char *format, ...)
 {
     int ret;
-    VIR_AUTOFREE(char *) str = NULL;
     VIR_AUTOFREE(char *) path = NULL;
     va_list ap;