sysctl: remove struct ctl_path

All usages of this struct have been removed from the kernel tree.

The struct is still referenced by scripts/check-sysctl-docs but that
script is broken anyways as it only supports the register_sysctl_paths()
API and not the currently used register_sysctl() one.

Fixes: 0199849a ("sysctl: remove register_sysctl_paths()")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>