Remove libcurl from Mac and Windows

[umireon]

We don't need to bundle libcurl on Mac and Windows because these OSes have their native HTTP clients.

The signature must include a callback because QTimer must be executed on the main thread.

[umireon]

I confirmed this works on Windows and Mac

[umireon]

I confirmed it works on Linux too.

[royshil]

i actually think this is not necessary
first - libcurl seems to work perfectly fine, and it's well supported cross-platform
second - the savings in build time are not that great, about 2 minutes. a much bigger savings is from the prebuilt static OpenCV libs
and also - using libcurl made us have a unified simple API for fetching data which is easier to maintain than platform-specific code.

let me know what you think..

[umireon]

I agree with your thought.

[umireon]

@royshil This change saves 6 minutes on Mac and this saves time more than pre-built OpenCV.
https://github.com/royshil/obs-backgroundremoval/actions/runs/5838854087
https://github.com/royshil/obs-backgroundremoval/actions/runs/5838815434

[royshil]

OK we can roll with this