Remove the --disable-carp option, keeping the CARP code always compiled in

Remove the --disable-carp option, keeping the CARP code always compiled in

this configure option was a relic from the time when CARP was very intrusive,
requiring --enable-carp to be compiled and then chainging parent relations.
The current CARP implementaiton is very non-intrusive and by default enabled
(but not active unless used in squid.conf). There isn't really any good reason
to why one should use --disable-carp today.