sql: Simplify logic for getting a TableDescriptor in scanNode.initTable
sql: Simplify logic for getting a TableDescriptor in scanNode.initTable

Now that `getTableLease` and `mustGetTableDesc` have the same signature,
this simplification is possible.

cockroachdb/cockroach,petermattis/cockroach,cockroachdb/cockroach,glycerine/cockroach,paperstreet/cockroach,arjun4084346/cockroach,paperstreet/cockroach,cockroachdb/cockroach,sunya123/cockroach,petermattis/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,petermattis/cockroach,maxlang/cockroach,sunya123/cockroach,sunya123/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,glycerine/cockroach,maxlang/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,paperstreet/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,petermattis/cockroach,glycerine/cockroach,sunya123/cockroach,arjun4084346/cockroach,paperstreet/cockroach,glycerine/cockroach,veteranlu/cockroach,petermattis/cockroach,sunya123/cockroach,cockroachdb/cockroach,glycerine/cockroach,maxlang/cockroach,sunya123/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,sunya123/cockroach,cockroachdb/cockroach,maxlang/cockroach,arjun4084346/cockroach,vivekmenezes/cockroach,glycerine/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,paperstreet/cockroach,veteranlu/cockroach,maxlang/cockroach,paperstreet/cockroach,maxlang/cockroach,vaibhavarora/cockroach,petermattis/cockroach,sunya123/cockroach,arjun4084346/cockroach,glycerine/cockroach,arjun4084346/cockroach,petermattis/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,veteranlu/cockroach,petermattis/cockroach,maxlang/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,paperstreet/cockroach,veteranlu/cockroach,glycerine/cockroach,maxlang/cockroach