webext: backends: Fix Cors for local Ogmios

[huynq-smartosc]

TL;DR: This fix to allow extension able to work with Chrome, when Backend Providers is local Ogmios/Kupo

Explanation:
When submit a tx to Ogmios, our extension will call to /?SubmitTransaction with header "Content-Type": "application/json" : Ref

But Ogmios only response back with Access-Control-Allow-Origin: *, but without Access-Control-Allow-Headers: content-type. This lead to Chrome rejected the cross-origin request POST.

Simply remove that Content-Type header will do the trick.