drivers: tee: optee: remove unused probe local variable

Removes local variable child in optee_probe() that is not used.

Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>