Remove Content-Type override for Mattermost OAuth login

What does this MR do?

It fixes #46810 (closed) by removing a workaround added in !19043 (merged). That workaround overrides the format of data received by the GitLab-Mattermost integration when it it logs into Mattermost.

As I mentioned here, I'm a bit confused as to why this workaround was needed since Mattermost still sends back the data with Content-Type: application/json as it did before. I did a good amount of testing, and everything worked fine.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

I tested this locally with Mattermost 5.10.0, and the GitLab-Mattermost integration was still able to create teams and slash commands in Mattermost.