Clean up file load.in in a test

This fixes the following test invocation:
./mtr --no-reorder parts.partition_special_myisam parts.partition_debug

This was broken ever since afd4b25d
a.k.a. mysql/mysql-server@bde21e58108a1f311d15088c50414bb43ab45d62

mysql-test/suite/parts/t/partition_special_myisam.test

@@ -74,6 +74,7 @@ COMMIT;
 
 --connection con3
 --reap
+--remove_file $MYSQLD_DATADIR/test/load.in
 DROP TABLE t1;
 
 --disconnect con3