media: v4l2-mediabus: Drop legacy V4L2_MBUS_CSI2_CHANNEL_* flags
The V4L2_MBUS_CSI2_CHANNEL_* flags are a legacy API. Only
V4L2_MBUS_CSI2_CHANNEL_0 is used, set in a single driver, and never
read. Drop those flags. Virtual channel information should be conveyed
through frame descriptors instead.