boards: mec15xxevb_assy6853: remove redundant I2C_INIT_PRIORITY

In this board I2C is required to talk to a GPIO expander (PCA95XX). By
default I2C devices are initialized with KERNEL_INIT_PRIORITY_DEVICE
(set to 50), and the PCA95XX driver is initialized with priority 70.
This means that there is no need to customize I2C priority as it will
always come before.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>