Remove runtime configuration options for CNI network backend:
Runtime options:
- Remove WithNetworkBackend() runtime option function
- Function allowed setting network backend programmatically
Flag handling:
- Remove --network-backend flag change detection
- Remove call to WithNetworkBackend() when flag changed
- Remove TODO comment about CNI plugins directory flag
The network backend configuration is now handled entirely by the
vendored common/libnetwork code, which will default to Netavark.
There is no longer any way to configure CNI as the network backend
through Podman's runtime initialization.
Note: libpod/info.go keeps existing NetworkBackend reporting logic
which will automatically report "netavark" as the only backend since
configuration defaults to netavark and cannot be changed to CNI.
Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com>