[Code Cleanup] : Small code cleanup in twophase.sql

Hi,

PFA small code cleanup in twophase.sql. Which contains a drop table
statement for 'test_prepared_savepoint'. Which, to me, appears to be
missing in the cleanup section of that file.

To support it I have below points:-

1) Grepping this table 'test_prepared_savepoint' shows occurrences
only in twophase.out & twophase.sql files. This means that table is
local to that sql test file and not used in any other test file.

2) I don't see any comment on why this was not added in the cleanup
section of twophase.sql, but drop for other two test tables are done.

3) I ran "make check-world" with the patch and I don't see any failures.

Kindly correct, if I missed anything.

Regards,
Nishant (EDB).