ta: storage: fix ta_storage_cmd_key_in_persistent() cleanup

Fixes the error cleanup path in two places where a created test object
was supposed to be removed.

Fixes: 90f23353 ("Test Persistent Object with keys")
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>