crimson/net: remove redundant std::move()

crimson/net: remove redundant std::move()

see https://en.cppreference.com/w/cpp/language/copy_elision

it also silences warning like:

src/crimson/net/ProtocolV2.cc:146:26: warning: redundant move in return
statement [-Wredundant-move]
  146 |       return std::move(bl);
      |                          ^

Signed-off-by: Kefu Chai <kchai@redhat.com>