Remove spaces API cookie roundtripper

[RedbackThomson]

Description of your changes

A cookie roundtripper was required when accessing any spaces admin APIs, since the mcp-api only accepted an SID cookie as authentication. As of https://github.com/upbound/managed-control-planes/pull/2006, the API can accept bearer tokens, instead.

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Running up ctx against dev

[RedbackThomson]

Pausing this for now as these changes haven't rolled out to staging or prod yet, but I wanted to get this PR out before I got lazy or forgot to do it.

It appears there's also a bug with the implementation in mcp-api because it doesn't work with the SSO session token, but it works when I give it a robot token. I'll follow up when that's fixed.