Remove Session appservice

It's no longer needed, and the naming can raise confusion.

gaphor/services/session.py

@@ -1,18 +0,0 @@
-from gaphor.abc import Service
-
-
-class Session(Service):
-    """Application service.
-
-    Get the active session.
-    """
-
-    def __init__(self, application):
-        self.application = application
-
-    def shutdown(self):
-        pass
-
-    def get_service(self, name):
-        assert self.application.active_session
-        return self.application.active_session.get_service(name)

pyproject.toml

@@ -130,7 +130,6 @@ type = "virtualenv"
 
 [tool.poetry.plugins."gaphor.appservices"]
 "event_manager" = "gaphor.core.eventmanager:EventManager"
-"session" = "gaphor.services.session:Session"
 "app_file_manager" = "gaphor.ui.appfilemanager:AppFileManager"
 
 [tool.poetry.plugins."gaphor.services"]