[#21382] test: clean up YSQL regress test yb_pg_timestamp

Summary:
yb_pg_timestamp regress test has a bunch of differences with the
original timestamp regress test because it tries to get sort order the
same by adding an extra column i int PRIMARY KEY.  Do it a smarter way
by using a serial column and making it the last column so that inserts
can remain the same was as in the original timestamp regress test.

Also, commit 71b760e badly adds an
ending DROP TABLE stmt to the original timestamp regress test, so remove
that here.
Jira: DB-10276

Test Plan:
On Almalinux 8:

    ./yb_build.sh fastdebug --gcc11 --java-test TestPgRegressTypesMisc

Close: #21382

Reviewers: kfranz

Reviewed By: kfranz

Subscribers: kfranz, yql

Differential Revision: https://phorge.dev.yugabyte.com/D33006