Don't install a custom error handler for hooks

Installing a custom error handler on every hook invocation has a high overhead,
and does not even correctly achieves what it sets out to achieve, which is to
flag hook function signature errors (and only hook function signature errors).
The "PHP way" is to simply increase the error reporting level for development
environments, which we do already.

Bug: T117553
Change-Id: Iba0138a6d0a0ddf839bc5a36e03cadb012e06f3c