[keyserver] remove duplicate content-type application/json header in call to assignBlobHolder

Summary:
Remove duplicate content type header in assign blob holder request to blob service.

In testing my farcaster bot, calls to createOrUpdatePublicLink were failing as it would eventually call assignBlobHolder with two duplicate content-type headers, resulting in a 400 Bad Request from the blob service.

We can fix this by specifying createRequestHeaders's includeContentType parameter as false

Linear Issue: https://linear.app/comm/issue/ENG-9986/running-into-an-issue-of-createorupdatepubliclink-failing-to-upload-a

Test Plan: In applying this change, the farcaster bot properly was able to upload invite link blobs without errors

Reviewers: varun, bartek, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D14091