lib: remove SAFI_RESERVED_4 and SAFI_RESERVED_5

lib: remove SAFI_RESERVED_4 and SAFI_RESERVED_5

SAFI values have been a major source of confusion over the last few
years. That's because each SAFI needs to be represented in two different
ways:
* IANA's value used to send/receive packets over the network;
* Internal value used for array indexing.

In the second case, defining reserved values makes no sense because we
don't want to index SAFIs that simply don't exist. The sole purpose of
the internal SAFI values is to remove the gaps we have among the IANA
values, which would represent wasted memory in C arrays. With that said,
remove these reserved SAFIs to avoid further confusion in the future.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>