cql3: remove unnecessary assert()

In cql3/, there was one call to assert() (not SCYLLA_ASSERT or
throwing_assert), and it was:

        const auto shard_num = smp::count;
        assert(shard_num > 0)

Rather than converting this assert() to throwing_assert() as I did in
previous patches, I decided to outright remove it: Seastar guarantees
that smp::count is not zero. Many other places in the code use
smp::count assuming that it is correct, no other place bothers to assert
it isn't zero.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

cql3/statements/modification_statement.cc

@@ -371,7 +371,6 @@ process_forced_rebounce(unsigned shard, query_processor& qp, const query_options
     // On the last iteration, re-bounce to the correct shard.
     if (counter != 0) {
         const auto shard_num = smp::count;
-        assert(shard_num > 0);
         const auto local_shard = this_shard_id();
         auto target_shard = local_shard + 1;
         if (target_shard == shard) {