Drop aliases in Wikibase main namespace

This is required for https://gerrit.wikimedia.org/r/#/c/165707/. Which is required for the HHVM switch.

Either this or we need to move all classes that use one of these aliases out of the `Wikibase` main namespace.

The problem is that the aliases are auto-loaded when the class that uses them is in the same namespace. Adding a `use` clause to use the actual class fails with somewhat confusing "Fatal error: Cannot use ... as ... because the name is already in use in ...". Which is correct. There already is a class (the alias) with that name in the same namespace.

To be discussed. This may need some more work. Maybe there is an other solution.