Remove libu{} target group

The semantics provided by libu{} is rarely required and as result was not
yet documented. However, if you are using it, the new way to achieve the
same result is to use both libue{} and libul{} explicitly, for example:

exe{foo}: libue{foo}
lib{foo}: libul{foo}

{libue libul}{foo}: cxx{*}

0 files changed, 0 insertions, 0 deletions