bgpd: Fix memory leak in SRv6 locator delete/unset

bgpd: Fix memory leak in SRv6 locator delete/unset

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 locators are deleted/unset in bgpd: when
an SRv6 locator is deleted/unset, all the chunks of the locator are
removed from the SRv6 locator chunks list (`bgp->srv6_locator_chunks`).
However, the memory allocated for the chunks is not freed.

This patch adds a call to the `srv6_locator_chunk_free()` function to
properly free the allocated memory when an SRv6 locator is removed or
unset.

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