Refactor to remove sysctl and iproute2 system files interfaces

[farshidtz]

This PR changes the setup and removal to work without accessing sysctl and iproute2 system files. This involves:

• Replacing the operations in _ipforward, _rt_tables, _border_routing with equivalent sysctl commands, executed in the oneshot setup service
• Removing the unused openthread->88 mapping from routing tables. See #14

In addition, for simplicity, nat44_install script is removed, and its contents are moved to the setup script.

The snap build from this PR was tested with:

• Ubuntu 23.10
• Chip Tool 1.1.0.1 snap
• 2x Nordic nRF52840 dongles

to commission and toggle the LED according to https://github.com/canonical/openthread-border-router-snap/wiki/Commission-and-control-a-Matter-Thread-device-via-the-OTBR-Snap