build: Don’t link backend libraries with libfolks-internal.la helper

Because libfolks-internal.la is a libtool helper library, and is never
actually installed, it gets statically linked into each backend. This
means that the GObject types in it (such as FolksSmallSet) get
duplicated between libfolks.so and each backend. This causes GObject
errors when the backends are loaded and attempt to re-register types
which already exist.

This is a partial revert of commits:
 • 150858a0237291d8269d222defcb82f54c1125cf
 • ef76b7cacb5cfdf7963e5c2a2fe3bc8c1c97838e
telepathy: Fix nullability of a function parameter

GBusNameVanishedCallbacks can legitimately have a NULL GDBusConnection
pointer passed to them. If the formal parameter isn’t marked as
nullable, Vala inserts an assertion which then fails and crashes.