media: rc: remove unused tx_resolution field

The tx_resolution field is never read. In theory you can imagine this
field being useful for detecting whether the transmitter has the
resolution for the message you are trying to send, but I am not aware of
any hardware where this could be an issue.

Just remove.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>