Remove escape-hatch for iptables rule for SCTP checksum fixup

[robmry]

Description

This escape-hatch was added in docker 28.0, and can be removed in 29.0 (or possibly a later "minor" release of 28.x)

Related to:

• https://github.com/moby/moby/issues/47952
• https://github.com/moby/moby/pull/48149

If the escape hatch DOCKER_IPTABLES_SCTP_CHECKSUM=1 isn't needed - remove it, and the code to set up the checksum mangling rule.