Manual pages: nsenter.1, unshare.1: remove repeated references to clone(2)

Manual pages: nsenter.1, unshare.1: remove repeated references to clone(2)

Back in commits f85b9777c2965671cd and 894efece9eb894, in the
description of each namespace type, I added repeated cross references
to clone(2). Drop these references. The Section 7 namespaces pages,
which are already noted in the nsenter(1) and unshare(1) manual pages,
provide much more relevant information. Furthermore, pointing the
reader at clone(2) is perhaps a little misleading, since the system
call underlying nsenter(1) is setns(2) and the system call underlying
unshare(1) is unshare(2).

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>