bgpd: Fix memory leak when an SRv6 SID is removed

bgpd: Fix memory leak when an SRv6 SID is removed

Running `bgp_srv6l3vpn_to_bgp_vrf` and `bgp_srv6l3vpn_to_bgp_vrf2`
topotests with `--valgrind-memleaks` gives several memory leak errors.
This is due to the way SRv6 SIDs are removed in bgpd: when
an SRv6 locator is deleted/unset, all the SIDs allocated from that
locator are removed from the SRv6 functions list
(`bgp->srv6_functions`),but the memory allocated for the SIDs is not
freed.

This patch adds a call to `XFREE()` to properly free the allocated
memory when an SRv6 SID is removed.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>