remove /post-sign-up needless redirect

[sqs]

After sourcegraph/sourcegraph#63373, the /post-sign-up indirection/redirection is no longer necessary.

Test plan

Manually test in debug ext host sign out then sign in

[chenkc805]

Let's wait to push this tomorrow, I know when @vdavid was working on this back in December we intentionally shipped it this way because this was a complicated flow. Here's some additional context:

• [Launch blocker] For new users creating an account from the IDE, they should go through only part of the Cody onboarding flow before getting pushed back to the editor #2054
• SSC: Add redirect from /post-sign-up to /cody/manage for users with the cody-pro feature flag sourcegraph#58870
• SSC: Handle returnTo params on /cody/manage sourcegraph#58854

[sqs]

@chenkc805 good to merge now? can you approve pls?

[chenkc805]

I don't know this code well enough. @vdavid knows this code but he's on pat leave.

Let me look at the git blame to see who's a good person to review

[chenkc805]

Looking at the git blame - both @philipp-spiess and @vdavid are unavailable to review this code. @dominiccooney - since you worked on the app-less signup flow last fall can I get your help reviewing this?