shadow: fix collision with coreutils (man groups.1.gz)

The `groups.1.gz` collides with one from coreutils. The code to fix this
was already present in expression, but wrongly assumes that share/man/man1
directory will be copied to `man` output after `installPhase`.

It turned out, that man directory is set at configure step, so we should
remove file from `man` output.