Remove plpgsql's RENAME declaration, which has bizarre and mostly nonfunctional

Remove plpgsql's RENAME declaration, which has bizarre and mostly nonfunctional
behavior, and is so little used that no one has been interested in fixing it.
To ensure that possible uses are covered, remove the ALIAS declaration's
arbitrary restriction that only $n identifiers can be aliased.

(We could alternatively make RENAME act just like ALIAS, but per discussion
having two different ways to do the same thing is probably more confusing than
helpful.)