Avoid possible regression test instability in timestamp.sql.

Concurrent autovacuum could result in a change in the order of the
live rows in timestamp_tbl.  While this would not happen with the
default autovacuum parameters, it's fairly easy to hit if
autovacuum_vacuum_threshold is made small enough to allow autovac
to decide to process this table.  That's a stumbling block for trying
to exercise autovacuum aggressively using the core regression tests.

To fix, replace an unqualified DELETE with a TRUNCATE.  There's a
similar DELETE just above (and no order-sensitive queries between),
so this doesn't lose any test coverage and might indeed be argued
to improve it.

Discussion: https://postgr.es/m/17428.1555348950@sss.pgh.pa.us

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