vlc-debian-unstable: remove libc++(abi)-dev

Both packages depend on libunwind-14-dev. However, gstreamer depends on
nongnu libunwind-dev. With both "providing" libunwind-dev, the nongnu
libunwind-dev is then removed. Without nongnu libunwind-dev, pkg-config
deems gstreamer not installed. Consequently VLC's configure also does
not find gstreamer, meaning that the gstdecode module won't be build.

As no project using vlc-debian-unstable depends on Clang's libc++ being
available (instead of libstdc++), simply remove libc(abi)++-dev to fix
building the gstdecode module in VLC CI.

Amends 6e8cb6bf
Fixes videolan/docker-images#4