Revert r106393; I do not find it acceptible for MediaWiki to ship by-default strangli...

Revert r106393; I do not find it acceptible for MediaWiki to ship by-default strangling it's deprecation notices.
Extension authors are supposed to be notified of deprecated code they are using that may disappear. This config variable was added so that devs who had an explicit reason to hide specific ranges of deprecation notices from MediaWiki (as opposed to all MediaWiki deprecation notices, or all php deprecation notices) not as a method of hiding notices that people are supposed to see from everyone.
Any dev who wants to should shoot themselves in the foot, that's their job. It is NOT our job to shoot everyone in the foot because some devs are too lazy to shoot themselves.