get rid of dead code in legitimize_root()

Combination of LOOKUP_IS_SCOPED and NULL nd->root.mnt is impossible
after successful path_init().  All places where ->root.mnt might
become NULL do that only if LOOKUP_IS_SCOPED is not there and
path_init() itself can return success without setting nd->root
only if ND_ROOT_PRESET had been set (in which case nd->root
had been set by caller and never changed) or if the name had
been a relative one *and* none of the bits in LOOKUP_IS_SCOPED
had been present.

Since all calls of legitimize_root() must be downstream of successful
path_init(), the check for !nd->root.mnt && (nd->flags & LOOKUP_IS_SCOPED)
is pure paranoia.

FWIW, it had been discussed (and agreed upon) with Aleksa back when
scoped lookups had been merged; looks like that had fallen through the
cracks back then.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>