remove support for deprecated kernel memory limit

kernel-memory limits are not supported in cgroups v2, and were obsoleted in
[kernel v5.4], producing a `ENOTSUP` in kernel v5.16. Support for this option
was removed in runc and other runtimes, as various LTS kernels contained a
broken implementation, resulting in unpredictable behavior.

We deprecated this option in [moby@b8ca7de], producing a warning when used,
and actively ignore the option since [moby@0798f5f].

Given that setting this option had no effect in most situations, we should
just remove this option instead of continuing to handle it with the expectation
that a runtime may still support it.

Note that we still support RHEL 8 (kernel 4.18) and RHEL 9 (kernel 5.14). We
no longer build packages for Ubuntu 20.04 (kernel 5.4) and Debian Bullseye 11
(kernel 5.10), which still have an LTS / ESM programme, but for those it would
only impact situations where a runtime is used that still supports it, and
an old API version was used.

[kernel v5.4]: https://github.com/torvalds/linux/commit/0158115f702b0ba208ab0
[moby@b8ca7de]: b8ca7de823
[moby@0798f5f]: 0798f5f5cf

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/info_unix.go

@@ -34,7 +34,6 @@ func (daemon *Daemon) fillPlatformInfo(ctx context.Context, v *system.Info, sysI
 	if v.CgroupDriver != cgroupNoneDriver {
 		v.MemoryLimit = sysInfo.MemoryLimit
 		v.SwapLimit = sysInfo.SwapLimit
-		v.KernelMemory = sysInfo.KernelMemory
 		v.KernelMemoryTCP = sysInfo.KernelMemoryTCP
 		v.OomKillDisable = sysInfo.OomKillDisable
 		v.CPUCfsPeriod = sysInfo.CPUCfs