Fix test_valid_server_encoding helper function.

Fix test_valid_server_encoding helper function.

Commit c67bef3f325 introduced this test helper function for use by
src/test/regress/sql/encoding.sql, but its logic was incorrect.  It
confused an encoding ID for a boolean so it gave the wrong results for
some inputs, and also forgot the usual return macro.  The mistake didn't
affect values actually used in the test, so there is no change in
behavior.

Also drop it and another missed function at the end of the test, for
consistency.

Backpatch-through: 14
Author: Zsolt Parragi <zsolt.parragi@percona.com>