bt2/clock-class.hpp: "clock class offset" -> "clock offset"

bt2/clock-class.hpp: "clock class offset" -> "clock offset"

It really is the offset a clock (clock class instance).

Also, "offset" -> "offset from origin".

This is in preparation of CTF 2 support (including libbabeltrace2
changes), where the concept of origin is more defined.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Id623085ae2194a452d2b9b07a8be17395324a5d5