Remove IDBEnvironment mixin

[Elchi3]

Yay, mixins.

Apparently the IDL was this:

interface mixin IDBEnvironment {
  [SameObject] readonly attribute IDBFactory indexedDB;
};

and it is now this:

partial interface mixin WindowOrWorkerGlobalScope {
  [SameObject] readonly attribute IDBFactory indexedDB;
};

So, BCD shouldn't have IDBEnvironment and also not WindowOrWorkerGlobalScope.indexeddb as that will soon be in a _globals folder. Proposing to just remove this file then.