Error codes for LIMIT and OFFSET

I was looking into adding new specific SQL:2008 error codes for invalid
LIMIT and OFFSET values (see attached patch), when I came across an
existing error code definition:

#define ERRCODE_INVALID_LIMIT_VALUE MAKE_SQLSTATE('2','2', '0','2','0')

This definition has been in our sources since error codes were first
added, but I don't find this code in the standard (it uses a
standard-space SQLSTATE code), and as far as I can tell, it hasn't been
actually used anywhere. Except that PL/pgSQL defines it in plerrcodes.h
(and Google shows that various other interfaces list it as well), but it
can never happen, I think.

What should we do here, if anything? Redefine
ERRCODE_INVALID_LIMIT_VALUE to the new SQL:2008 code? Or remove the
whole thing (assuming that no PL/pgSQL code actually referes to it)?