opts: remove unused RuntimeOpt

This type was copied from the daemone in 8b725e1,
but actually unused on the CLI side, so we can remove it here.

The original code is still present in the daemon at
https://github.com/moby/moby/blob/7b9275c0da707b030e62c96b679a976f31f929d3/opts/runtime.go

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah