pgsql: Revert pg_bind_textdomain_codeset to a existant-but-empty

Log Message:
-----------
Revert pg_bind_textdomain_codeset to a existant-but-empty function when
ENABLE_NLS is not defined, for better compatibility of the backend with
modules compiled the other way.

Per note from Tom after my previous commit.

Modified Files:
--------------
    pgsql/src/backend/utils/mb:
        mbutils.c (r1.81 -> r1.82)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/mbutils.c?r1=1.81&r2=1.82)
    pgsql/src/include/mb:
        pg_wchar.h (r1.86 -> r1.87)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/mb/pg_wchar.h?r1=1.86&r2=1.87)