[9.x] Remove redundant Doctrine method on schema builder

[bakerkretzmar]

Follow-up to #40119 and #40303. Removes Schema::registerCustomDoctrineType(), the correct way to register custom types is with DB::registerDoctrineType() or in the config, and if for some reason someone still wants to do it this way they can use Schema::getConnection()->registerDoctrineType().

[taylorotwell]

Do we need a PR to the 9.x upgrade guide for this? (laravel/docs master branch)

[bakerkretzmar]

Good call, will do 👍🏻