Delete allmydata.version_checks and related functionality

It is not Tahoe-LAFS' job to manage package installation in this way.
Instead, we can declare our dependencies in setup.py and rely on installation
management tools and packagers to create a suitable execution environment.

Making this statement in the past required going much further out on a limb
than it does today.  This code has served its purpose and can now be retired.

src/allmydata/client.py

@@ -512,7 +512,6 @@ def create_introducer_clients(config, main_tub, _introducer_factory=None):
             config.nickname,
             str(allmydata.__full_version__),
             str(_Client.OLDEST_SUPPORTED_VERSION),
-            list(node.get_app_versions()),
             partial(_sequencer, config),
             introducer_cache_filepath,
         )