liblog: remove obsolete comment

The log_time struct satisfies all of the requirements for an
implicitly created copy constructor to be present, so not defining one
here does not have any real effect.

We don't want to delete the copy constructor for the rationale given
either; modern C++ favors passing small types by value instead of by
reference as the compiler has more opportunity for optimization in
that case.  That's especially true here, where the size of this struct
is the size of a pointer on 64 bit systems.

Test: the copy constructor exists for log_time
Change-Id: Id314ca7729f4b1ca02adb6c7f0ae759b22be2a5c