Trebuchet: Disable/Hide Dock
There does not seem to be a way to do that in Trebuchet on CyanogenMod (when not on CyanogenMod you can do it in the Trebuchet Settings).

As an alternative you could use nova launcher, it is free and the dock is removeable (even on CyanogenMod).

There may be many more launcher providing this feature, but nova is tho only one I used on CyanogenMod.