client/SyntheticClient.cc: remove dead code

Fix for:

const: At condition do_sync, the value of do_sync must be equal to 0.
dead_error_condition: The condition do_sync cannot be true.
CID 716893 (#1 of 1): Logically dead code (DEADCODE)
dead_error_begin: Execution cannot reach this statement op.OSDOp()

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
test_librbd.cc: fix compiler warning

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>