[development] Eliminate hooks entirely in Drupal 6

[Moshe Weitzman]

Earl Miles wrote:
> Gerhard Killesreiter wrote:
>> Is there any reason to allow non-standard entries in the first place?
> 
> Yes. Because if we do, we can let non-module files utilize these 
> callbacks. That could be a benefit.
> 

furthermore, we will allow hook ordering as chx suggested and has shown in 
his sandbox. scanned hooks will just get the default weight, like today.