netfilter: merge udp and udplite conntrack helpers

udplite was copied from udp, they are virtually 100% identical.

This adds udplite tracker to udp instead, removes udplite module,
and then makes the udplite tracker builtin.

udplite will then simply re-use udp timeout settings.
It makes little sense to add separate sysctls, nowadays we have
fine-grained timeout policy support via the CT target.

old:
 text    data     bss     dec     hex filename
 1633     672       0    2305     901 nf_conntrack_proto_udp.o
 1756     672       0    2428     97c nf_conntrack_proto_udplite.o
69526   17937     268   87731   156b3 nf_conntrack.ko

new:
 text    data     bss     dec     hex filename
 2442    1184       0    3626     e2a nf_conntrack_proto_udp.o
68565   17721     268   86554   1521a nf_conntrack.ko

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>