Remove 'friend class EntryFilter' from SessionManagerBase.

Fixes build on MSVC.

Apparently if you declare a friend in a class inside a namespace:
namespace NS {
 class Foo {
  friend class Bar;
 }
}
MSVC assumes you're referring to NS::Bar instead of ::Bar, unless Bar
was already (forward-)declared before.

But it turns out his friendship declaration isn't even necessary,
since the code builds without it.