Remove $wgExceptionHooks

Remove $wgExceptionHooks

Skip deprecation period because it is very unlikely that anyone used this:
it does not appear anywhere on gerrit/git, no nontrivial google hits,
the documentation has been flat out wrong for 9 years and no one
noticed it, and the whole feature is fairly useless as you need to declare
it separately for every single exception class you expect.

Change-Id: I85844a238d3135d05eeba10331149624b04bafe2