remove upsert example from docs

Attached is a patch to remove the upsert example from the pl/pgsql
documentation. It has a serious bug (see:
http://www.spinics.net/lists/pgsql/msg112560.html) which is nontrivial
to fix. IMNSHO, our code examples should encourage good practices and
style.

The 'correct' way to do race free upsert is to take a table lock first
-- you don't have to loop or open a subtransaction. A high
concurrency version is nice but is more of a special case solution (it
looks like concurrent MERGE might render the issue moot anyways).

merlin