Remove ancient PL/pgsql line numbering hack.

Remove ancient PL/pgsql line numbering hack.

While this hack arguably has some benefit in terms of making PL/pgsql's
line numbering match the programmer's expectations, it also makes
PL/pgsql inconsistent with the remaining PLs, making it difficult for
clients to reliably determine where the error actually is.  On balance,
it seems better to be consistent.

Pavel Stehule