of/overlay: Remove unused variable

Commit 3e7f7626fd49a ("of/overlay: Do not generate duplicate nodes") removed
the only use of the 'grandchild' variable, which leads to the following build
warning:

drivers/of/overlay.c: In function 'of_overlay_apply_single_device_node':
drivers/of/overlay.c:89:31: warning: unused variable 'grandchild' [-Wunused-variable]
  struct device_node *tchild, *grandchild;
                               ^

Remove this unused variable.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Rob Herring <robh@kernel.org>