mailbox: stm32-ipcc: remove duplicate error message

platform_get_irq_byname already prints an error message if the requested
irq was not found. Don't print another message in the driver.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>