[YCQL] unreachable code

[maddin200]

Jira Link: DB-4454

Description

/src/yb/yql/cql/ql/exec/executor.cc line 1627

code:
case ExplainPlanPB::PLAN_NOT_SET: {
return exec_context_->Error(tnode, ErrorCode::EXEC_ERROR);
break; // <-- unreachable
}