Remove some ancient and unmaintained encoding-conversion test cruft.

In commit 92119191 I claimed that we weren't testing encoding
conversion functions, but further poking around reveals that we did
have an equivalent though hard-wired set of tests in conversion.sql.
AFAICS there is no advantage to doing it like that as compared to letting
the catalog contents drive the test, so let the opr_sanity addition stand
and remove the now-redundant tests in conversion.sql.

Also, remove some infrastructure in src/backend/utils/mb/conversion_procs
for building conversion.sql's list of tests.  That was unmaintained, and
had not corresponded to the actual contents of conversion.sql since 2007
or perhaps even further back.