bgpd: Release SID on router deletion (backport #17913)

[mergify]

Closes #15753 .

We need to call sid_unregister() in order to release a SID, which is called from the delete_vrf_tovpn_sid() and vpn_leak_postchange(). The problem is that vpn_leak_postchange() seems not to be called when we delete a router. I only see calls to vpn_leak_prechange() in bgp_delete(). It also seems that calling the whole postchange there would be incorrect.

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This is an automatic backport of pull request #17913 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of f3680ab has failed:

On branch mergify/bp/stable/10.0/pr-17913
Your branch is up to date with 'origin/stable/10.0'.

You are currently cherry-picking commit f3680ab41.
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Unmerged paths:
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	both modified:   bgpd/bgpd.c

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Cherry-pick of 90fe717 has failed:

On branch mergify/bp/stable/10.0/pr-17913
Your branch is ahead of 'origin/stable/10.0' by 1 commit.
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You are currently cherry-picking commit 90fe71735.
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Unmerged paths:
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	deleted by us:   tests/topotests/bgp_srv6_sid_reachability/test_bgp_srv6_sid_reachability.py

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