Remove legacy v2 `signin/signout` session services from influxdb3-python

[bednar]

The Python InfluxDB 3 client was derived from the v2 client and still contains legacy username/password session authentication flow. This includes SigninService, SignoutService, /api/v2/signin, /api/v2/signout, and automatic cookie handling in the generated-style ApiClient.

InfluxDB 3 client authentication should be token-based for write/query operations. The session-based signin/signout flow is not part of the supported v3 client surface and adds unnecessary complexity.

Scope
Remove or deprecate the legacy v2 session authentication path:

• Remove SigninService and SignoutService exports from package init.py files.
• Remove automatic _signin() / _signout() calls from write_client/_sync/api_client.py.
• Remove imports of SigninService and SignoutService.
• Remove username/password session cookie logic that calls /api/v2/signin and /api/v2/signout.
• Keep token-based authentication and auth_scheme behavior unchanged.