bgpd: Don't check for NULL when removing SRv6 SIDs

bgpd: Don't check for NULL when removing SRv6 SIDs

When an SRv6 locator is unset, all the SRv6 SIDs allocated from the
locator are removed. Before freeing the memory allocated for an SRv6
SID, we check if the pointer to the SID is `NULL`.

However, checking for `NULL` before freeing memory is useless.

This PR aims to improve the code's readability by removing the
useless `NULL` checks.

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