Remove unused SharedSession configuration helper

[Faerkeren]

Problem

SharedSession is dead code.

It is defined at src/haclient/config.py:131-150, but no production code imports or instantiates it. Session ownership is handled directly inside AiohttpRestAdapter and AiohttpWebSocketAdapter.

The only reason config.py imports aiohttp is this unused class.

Suggested Fix

Delete SharedSession and the aiohttp import from src/haclient/config.py.

If a shared-session abstraction is still desired, introduce it where it is actually used by both adapters and add tests around ownership behavior.

Rationale

Dead abstractions make the architecture look more complicated than it is and invite contributors to build on code that has no runtime role.