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