Stop building private_browsing.exe on Windows

Between 102 and 115, Firefox introduced a new menu item for PBM on Windows.

I can see a few new assets:

• content/about-logo-private.png (and also 2x): it's used in the new PBM window home, so I think we don't need them
• pbmode.ico: I think it's used both for the context menu and for the taskbar. Probably copying the regular icon is enough for us, since we're using PBM by default. Otherwise, we'll need new assets, since they contain Firefox's logo
• PrivateBrowsing_70.png and PrivateBrowsing_150.png: they are used in the start menu. They're like pbmode.ico with some transparent padding
• private_browsing.VisualElementsManifest.xml: file to manage the PrivateBrowsing_70.png and PrivateBrowsing_150.png. It also includes a background color definition (#14171a for PBM, vs. #20123a for regular FF)

I think we could use our usual icons, for starters (i.e., unblock Tor Browser + Mullvad Browser builds).

Then, we should see if always-on PBM disables these menus (I don't think so), and/or if we want to remove them, and keep only the regular ones for Tor/Mullvad Browser.

FWIW, Firefox also ships a new private_browsing.exe on Windows (and we ship it too). It's a consumer of the .ico file, in addition to the start menu entry.

Finally, I'm opening the issue on Tor Browser, but the same problems affect Mullvad Browser, too. We can either solve them in the same way, or open a new issue for Mullvad Browser and discuss there separately.

/cc @donuts @ruihildt