Remove the custom_variable_classes parameter.

This variable provides only marginal error-prevention capability (since
it can only check the prefix of a qualified GUC name), and the consensus
is that that isn't worth the amount of hassle that maintaining the setting
creates for DBAs.  So, let's just remove it.

With this commit, the system will silently accept a value for any qualified
GUC name at all, whether it has anything to do with any known extension or
not.  (Unqualified names still have to match known built-in settings,
though; and you will get a WARNING at extension load time if there's an
unrecognized setting with that extension's prefix.)

There's still some discussion ongoing about whether to tighten that up and
if so how; but if we do come up with a solution, it's not likely to look
anything like custom_variable_classes.