mon/MonClient: delete MonConnection move assignment

3e885abc2ae ("mon/MonClient: add assertions for monc lock in
MonConnection") added a `ceph::mutex& monc_lock` reference member to
MonConnection, which makes the explicitly defaulted move assignment
operator implicitly deleted.  clang (-Wdefaulted-function-deleted,
promoted by -Werror) rejects that, so every make check / API build on
main fails since that merge:

  src/mon/MonClient.h:70:18: error: explicitly defaulted move assignment
  operator is implicitly deleted [-Werror,-Wdefaulted-function-deleted]

Nothing move-assigns a MonConnection (pending_cons/active_con only ever
move-construct), so spell out what the compiler already decided.

Fixes: 3e885abc2ae3ae50fe443046594a01ece386657b
Signed-off-by: David Galloway <david.galloway@ibm.com>
Assisted-by: Claude Fable 5 (claude-fable-5)