libcxx[abi]: remove

libcxx[abi]: remove

This is a neat project, but offers no benefit to OpenWrt. The initial
reason for it was to be a replacement for libstdcpp as it is smaller
and lacks compatibility for C++98. Unfortunately, compiling several
packages with it results in larger ipk sizes.

While not a member of the packages feed, this will be moved to
packages-abandoned to keep it somewhere.

Signed-off-by: Rosen Penev <rosenp@gmail.com>