Fix unused variable warning in if_pfsync.c

With clang 15, the following -Werror warning is produced:

sys/netpfil/pf/if_pfsync.c:2153:9: error: variable 'sent' set but not used [-Werror,-Wunused-but-set-variable]
        int i, sent = 0;
               ^

The 'sent' variable was used in the for loop later in the
pfsync_bulk_update() function, but refactoring in 4fc65bcbe3fb7 got rid
of it. Remove the variable since it no longer serves any purpose.

MFC after: 3 days