zebra: Fix memory leak in SRv6 locator delete

zebra: Fix memory leak in SRv6 locator delete

Running `srv6_locator` topotest with `--valgrind-memleaks` gives several
memory leak errors. This is due to the way SRv6 locators are deleted:
when an SRv6 locator is deleted, it is removed from the SRv6 locators
list (`srv6->locators`), but the memory allocated for the SRv6 locator
is not freed.

This patch adds a call to the `srv6_locator_free()` function to properly
free the allocated memory when an SRv6 locator is removed.

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