[TODO] Remove manifest endpoint and related /server/version.py

[augustjohnson]

Description

There is a non-functional and generally deprecated (and historically unused as far as we can tell) endpoint called /manifest. It is built on logic in server/version.py, which is itself flawed.

What needs doing?
Delete the routes, and supporting files.
Test changes. Clean up any problems.

How do we know it's done?
Unused/unnecessary data is removed.
Tests run successfully.