pgsql: Remove some ancient and unmaintained encoding-conversion test cr

Remove some ancient and unmaintained encoding-conversion test cruft.

In commit 921191912c48a68d 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.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/419400c5da738d86c87e903a3d1924ff365bf203

Modified Files
--------------
src/backend/utils/mb/conversion_procs/Makefile     |   20 -
.../utils/mb/conversion_procs/regress_epilogue     |    5 -
.../utils/mb/conversion_procs/regress_prolog       |   25 -
src/test/regress/expected/conversion.out           |  918 +-------------------
src/test/regress/sql/conversion.sql                |  269 +-----
5 files changed, 8 insertions(+), 1229 deletions(-)