Clean up test output files for rocksdb.bypass_select_unsupported

Summary:
Running the test on repeat fails because output files were not cleaned
up after the run.

Squash with D36765784

Reviewed By: bladepan

Differential Revision: D45715828

fbshipit-source-id: a9a091b

Author: hermanlee

mysql-test/suite/rocksdb/t/bypass_select_unsupported.inc

@@ -484,3 +484,7 @@ drop table t3;
 drop table t4;
 drop table t5;
 drop table t6;
+if ($bypass_fallback != 0) {
+--remove_file $datadir/select.out
+--remove_file $datadir/select.dump
+}