ipv4: drop fragmentation code from ip_options_build()

Since v2.5.44 and addition of ip_options_fragment()
ip_options_build() does not render headers for fragments
directly. @is_frag is always 0.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>