pgsql: Fix cross-version upgrade tests.

Fix cross-version upgrade tests.

TestUpgradeXversion knows how to make the main regression database's
references to pg_regress.so be version-independent.  But it doesn't
do that for plperl's database, so that the C function added by
commit b7e3a52a8 is causing cross-version upgrade test failures.
Path of least resistance is to just drop the function at the end
of the new test.

In <= v14, also take the opportunity to clean up the generated
test files.

Security: CVE-2024-10979

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/256e34653aadd3582b98411d7d26f4fbb865e0ec

Modified Files
--------------
src/pl/plperl/GNUmakefile              | 1 +
src/pl/plperl/input/plperl_env.source  | 3 +++
src/pl/plperl/output/plperl_env.source | 2 ++
3 files changed, 6 insertions(+)