qb: removed error from builder ToCql function

Builder is not in a position to validate the statement and the effort is
duplicated when the query is build. The checked errors are simple and
do not occur most of the time.

qb/insert_test.go

@@ -53,10 +53,7 @@ func TestInsertBuilder(t *testing.T) {
 	}
 
 	for _, test := range table {
-		stmt, names, err := test.B.ToCql()
-		if err != nil {
-			t.Error("unexpected error", err)
-		}
+		stmt, names := test.B.ToCql()
 		if diff := cmp.Diff(test.S, stmt); diff != "" {
 			t.Error(diff)
 		}