reactor: Drop a few redundant calls to engine()
reactor: Drop a few redundant calls to engine()

In one case engine() is just "this", in the others we can use
this_shard_id().

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200323160532.37528-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,cloudius-systems/seastar