conf/defaultsetup.conf: Drop TCLIBCAPPEND

We've been able to run musl and glibc builds in the same TMPDIR for
many years and a separate directory is not required. Most distros disable
this value for that reason.

Drop support for the variable to make it clear and easy for distros
to be able to set multiconfigs which behave consistently for distros
which do and don't clear it by dropping it entirely.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>