cmLinkItem: Remove redundant cmLinkImplItem class

The cmLinkImplItem class used to contain some additional members
to support the CMP0027 policy. Since the CMP0027 policy and those
associated members were removed, the cmLinkImplItem class has been
a plain wrapper around cmLinkItem with no additional members.
It is no longer needed and cmLinkItem can be used directly wherever
cmLinkImplItem was used previously.